Optical System and Assembly Method

ABSTRACT

An optical system which includes some or all of the following parts: a laser light source which illuminates a spatial light modulator such that optical characteristics are preserved; a stereoscopic display which has a polarization-switching light source; a stereoscopic display which includes two infrared lasers, two optical parametric oscillators, and six second harmonic generators; two light sources processed by two parts of the same spatial light modulator; a method of assembly using an alignment plate to align kinematic rollers on a holding plate; an optical support structure which includes stacked, compartmented layers; a collimated optical beam between an optical parametric oscillator and a second harmonic generator; a laser gain module with two retroreflective mirrors; an optical tap which keeps the monitored beam co-linear; an optical coupler which includes an optical fiber and a rotating diffuser; and an optical fiber that has a core with at least one flat side.

BACKGROUND OF THE INVENTION

Movie theaters have traditionally relied on projectors which use anarc-lamp as the source of light. This is a highly inefficient source forprojecting an image onto a screen due to the extended size of thesource, its low brightness, its broadband light, and the relativelyshort lifetime of the lamp itself. The cost of ownership to theaterowners is large due to the frequent lamp changes required, the cost ofelectricity to operate the multi kilowatt lamps as well as the coolingrequired to keep the projector room at a normal operating temperature.

The move to digital cinema further reduces the efficiency of the lampdue to the requirement that the light needs to be separated intocomponent red/green/blue bands to illuminate three modulation arrays ina uniform manner before recombining the beams for projection. Finally,3-D digital cinema requires even more light as the projected image mustbe alternated between scenes for the left and right eyes which reducesthe apparent brightness of the image on the screen.

Red, Green, Blue solid state lasers seem to be a viable solution tothese problems while reducing the total cost of ownership. The pumplasers have very long rated lives, on the order of 20,000 hours,eliminating the need for lamp replacements. Semiconductor pump lasershave high electrical to optical efficiency meaning much less heatgeneration for a given output power as compared to arc lamps. The lasersare narrow-band which makes them easier to separate with little lightloss. They also are much brighter sources which allows high opticalthroughput. Finally lasers are typically polarized which provides a bigpower advantage for some 3-D display technologies.

Solid-state lasers are not without problems, however. The typical highpurity TEM00 Gaussian spatial mode is difficult to convert to a uniformsource over the digital modulators. The narrow line width of a laserleads to speckle patterns over the viewed image which may lead to anunacceptable loss of image quality.

The optical designs of most digital image projectors use spatial lightmodulators (SLMs) to switch each pixel on and off in order to create avisual image. The SLMs may be reflective, such as liquid crystal onsilicon (LCOS) devices and digital micromirror devices (DMDs), or may betransmissive such as liquid crystal display (LCD) panels.

Some of the commonly used components of laser optical systems includeoptical parametric oscillators (OPOs), and laser gain modules. OPOs maybe used to generate multiple wavelengths of laser light from one pumplaser beam. In the OPO, parametric amplification in a nonlinear crystalconverts the pump laser wavelength into two more wavelengths of light,so an optical system with one pump laser and one OPO may produce attotal of three wavelengths of visible light which are useful forapplications such as full-color digital image projection. Laser gainmodules are used to optically amplify laser light. A laser is used topump a gain slab which is composed of a gain medium. Optical energy istransferred from the pump beam into a main beam of light which is alsotraveling through the gain slab.

Many other optical components are used in laser light sources,projectors, and optical systems in general. Commonly used abbreviationsare as follows: ultra-high performance (UHP) lamp, polarizingbeamsplitter (PBS), dichroic beamsplitter (DBS), second harmonicgeneration (SHG) unit, total-internal-reflection (TIR) prism,antireflection (AR) coating, neodymium-doped yttrium lithium fluoride(Nd:YLF) laser, neodymium-doped yttrium aluminum perovskite (Nd:YAP)laser, neodymium-doped yttrium lithium fluoride (Nd:YLF) laser, lithiumtriborate (LBO) crystal, short wave pass (SWP) filter, long wave pass(LWP) filter, subminiature A (SMA) connector, and light emitting diode(LED). Some of the concepts used in optics and their abbreviations arefull-width half maximum (FWHM) bandwidth, angle of incidence (AOI), andultraviolet (UV) light. The organizations and industry standards thatapply include the Digital Cinema Initiative (DCI), the CommissionInternationale de l'Eclairage (CIE), and the InternationalTelecommunication Union Radiocommunication (ITU-R) Recommenation 709(Rec. 709).

Assembly and alignment of optical systems generally require that theoptical components be placed in a desired position with high accuracyand that the components be held in that position throughout shipping andover the lifetime of the product. In manufacturing quantities ofthousands or more, conventional optical assembly techniques do notprovide cost-effective methods for assembling optical devices with hightolerances such as complex laser systems. Optical systems are typicallyassembled on the upper surface of a flat optical support structure. Eachoptical component of the optical system is aligned and attached to theflat optical support structure in its desired location.

A stereoscopic projector forms still or moving images that can be seenin three dimensions. Stereoscopic projection systems may be formed byusing polarized light to form distinct images for the left eye and theright eye. These images simulate the images that would be seen in anactual three-dimensional scene. One polarization state is used for theleft-eye image and the orthogonal polarization is used for the right-eyeimage. Glasses with polarizing filters are used to allow the left imageto pass through to the left eye and the right image to pass through tothe right eye, while blocking the left image from reaching the righteye, and blocking the right image from reaching the left eye. In otherwords, the image for the left eye is directed to the left eye and not tothe right eye, whereas the image for the right eye is directed to theright eye and not to the left eye.

Instead of using polarized light, stereoscopic left and right images maybe formed by using spectral selection, for example as described in U.S.Pat. No. 6,283,597, the complete disclosure of which is incorporatedherein by reference. In the spectral selection method, first wavelengthbands of red, green, and blue are passed to the left eye, and secondwavelength bands of red, green, and blue are passed to the right eye.The first bands and second bands are distinct so that there is little orno overlap between the first and second bands.

Stereoscopic projection systems can be characterized as one of threebasic types: (1) time-sequential projection that uses one SLM per colorand alternately shows left eye images and right eye images in rapidsequence, (2) simultaneous projection that uses two SLMs per color, onefor the left eye images and one for the right eye images, and (3) splitimage projection, where there is only one SLM per color, and the leftand right eye images are formed simultaneously on separate parts orpixels of the single SLM.

In summary, the main problems facing digital image projectors areproviding a bright image with a long operation lifetime, especially inthe case of stereoscopic systems, and providing an alignment andassembly method that is feasible in full-scale production.

SUMMARY OF THE INVENTION

In embodiments, in one aspect, an optical system including a laser lightsource and an SLM where the SLM includes a liquid crystal material. Thelaser light source emits light only in a range of wavelengths thatpreserves an optical characteristic of the SLM.

In embodiments, in one aspect, a stereoscopic display system including apolarization-switching light source and a polarization-preservingprojector which is illuminated by the polarization-switching lightsource.

Implementations may include one or more of the following features. Thepolarization-preserving projector may form a left-eye digital image anda right-eye digital image, and the polarization state of thepolarization switching light source may be changed in synchronizationwith an alternating projection of the left-eye digital image and theright-eye digital image.

In embodiments, in one aspect, a stereoscopic projection systemincluding a first infrared laser, a first gain module that amplifies thelight beam from the first infrared laser, a first SHG that frequencydoubles the light beam from the first gain module, a first OPO thatparametrically amplifies the light beam from the first SHG, a second SHGthat frequency doubles the first light beam from the first OPO; a thirdSHG that frequency doubles the second light beam from the first OPO, asecond infrared laser, a second gain module that amplifies the lightbeam from the second infrared laser; a fourth SHG that frequency doublesthe light beam from the second gain module, and the like. Part of thelight beam from the first SHG passes through the first OPO to form aremaining light beam which is green. The light beam from the second SHGis red, the light beam from the third SHG is blue, and the light beamfrom the fourth SHG is a second color of green.

Implementations may include one or more of the following features. Theremaining light beam, the light beam from the second SHG, and the lightbeam from the third SHG may combine to form an image that is directed toone eye of the viewer and is not directed to the other eye of theviewer. There may be a switch that switches the light beam from thefirst SHG, a second OPO that parametrically amplifies the light beamfrom the first SHG, a fifth SHG that frequency doubles the first lightbeam from the second OPO, a sixth SHG that frequency doubles the secondlight beam from the second OPO. The switch may send the light beam fromthe first SHG alternately to the first OPO and the second OPO, and thelike. The light beam from the fifth SHG may be a second color of redlight, and the light beam from the sixth SHG may be a second color ofblue light. There may be a third infrared laser, a third gain modulethat amplifies the light beam from the third infrared laser, a fifth SHGthat frequency doubles the light beam from the third gain module; asecond OPO that parametrically amplifies the light beam from the fifthSHG, a sixth SHG that frequency doubles the first light beam from thesecond OPO, a seventh SHG that frequency doubles the second light beamfrom the second OPO, and the like. The light beam from the sixth SHG mayhave a second color of red light, and the light beam from the seventhSHG may have a second color of blue light.

In embodiments, in one aspect, an optical system including a first lightsource, a second light source; and an SLM. The first light source has afirst optical output which is processed by a first part of the SLM andthe second light source has a second optical output which is processedby a second part of the SLM.

Implementations may include one or more of the following features. Thefirst light source may have an etendue lower than 0.1 mm² sr. The firstpart of the SLM may be used to form an image for the left eye of theviewer and the second part of the SLM may be used to form an image forthe right eye of the viewer. The first optical output may include afirst wavelength band and the second optical output may include a secondwavelength band and the first wavelength band may be different than thesecond wavelength band.

In embodiments, in one aspect, a method of assembly including the stepsof placing an alignment plate on a holding plate, inserting a roller anda holding block into the alignment plate, fastening the holding block tothe holding plate to hold the roller, fastening the roller to theholding plate, removing the alignment plate, mating an optical module tothe roller on the holding plate, and the like.

Implementations may include the following feature. The final alignmentmay be achieved without further adjustments.

In embodiments, in one aspect, an optical support structure includingfirst and second compartmented support structures adapted to supportoptical modules. The second compartmented support structure is stackedon top of the first compartmented support structure.

Implementations may include one or more of the following features. Theremay be a first compartment in the first compartmented support structure,a second compartment in the second compartmented support structure, ahole between the first compartment and the second compartment thatallows a beam of light to pass through, and the like. There may be athird compartment in the second support structure, and a hole betweenthe second compartment and the third compartment that allows a beam oflight to pass through.

In embodiments, in one aspect, an optical system including an OPO, anSHG, a first lens which passes light between the OPO and the SHG, asecond lens which passes light between the OPO and the SHG, a third lenswhich passes light between the OPO and the SHG, and the like.

Implementations may include the following feature. The first lens maypass a collimated beam segment to the second lens.

In embodiments, in one aspect, an apparatus including a laser gain slabwhich carries a main laser beam, a pump laser which optically pumps thelaser gain slab, a retroreflective minor positioned adjacent to thelaser gain slab, and the like. The retroreflective minor reflects themain laser beam.

In embodiments, in one aspect, an optical tap including a first plate, asecond plate, a detector, and the like. A first beam of light enters thefirst plate and a small fraction of the first beam of light is reflectedto the detector. A second beam of light exits the first plate and entersthe second plate. The second plate shifts the second beam of light to beco-linear with the first beam of light.

Implementations may include the following feature. The first plate maybe an uncoated plate of glass.

In embodiments, in one aspect, an optical coupler including an opticalfiber and a despeckler. A laser light beam illuminates the opticalfiber, the output from the first optical fiber illuminates anintegrating rod, and the output from the integrating rod illuminates adigital image projector.

Implementations may include one or more of the following features. Theremay be a second optical fiber, another laser light beam whichilluminates the second optical fiber, the output from the second opticalfiber may illuminate the despeckler, and the like. There may be a thirdoptical fiber and a third laser light beam which illuminates the thirdoptical fiber, and the output from the third optical fiber mayilluminate the despeckler. The first laser light beam may be red, thesecond laser light beam may be green, and the third laser light beam maybe blue. The first optical fiber may be attached to the second opticalfiber to form an optical fiber bundle.

In embodiments, in one aspect, an optical system including a laser lightsource, an optical fiber with a core, and a digital image projector. Thelaser light source illuminates the core, the core illuminates thedigital image projector, and the core has at least one flat side.

Implementations may include one or more of the following features. Thecore may have a rectangular cross section. The polarization direction ofthe laser light source may be oriented orthogonal to the flat side ofthe core.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram of an optical system which includes at leastone laser light source, at least one coupler, and at least one set ofSLMs;

FIG. 2 is a top view of a projector system using LCOS SLMs;

FIG. 3 is a top view of a projector system using LCD SLMs;

FIG. 4 is a graph of the spectrum of a xenon lamp;

FIG. 5 is a graph of the shortwave spectrum of a xenon lamp;

FIG. 6 is a graph of the safe blue spectrum of a xenon lamp;

FIG. 7A is a graph of the spectrums of two narrow-band blue lasers;

FIG. 7B is a graph of the spectrum of a wide-band blue laser;

FIG. 8 is a color chart of two xenon-projector color gamuts compared tothe DCI standard;

FIG. 9 is a color chart of two laser-projector color gamuts compared tothe DCI standard;

FIG. 10 is a color chart of a laser-projector color gamut compared tothe Rec. 709 standard;

FIG. 11 is a graph of the spectrum of a UHP lamp;

FIG. 12 is a side view of a stereoscopic display system;

FIG. 13 is a side view of a stereoscopic display system with apolarization-switching light source;

FIG. 14 is a top view of a polarization-preserving projector;

FIG. 15 is a side view of a polarization-switching light source;

FIG. 16 is a front view of a rotating disk in a polarization switch;

FIG. 17 is a flowchart of a method of stereoscopic projection;

FIG. 18 is a top view of stereoscopic projection;

FIG. 19 is a block diagram of a projector light engine;

FIG. 20 is a block diagram of a laser light system based on two lasers;

FIG. 21 is a block diagram of a laser light system based on threelasers;

FIG. 22 is a flowchart of a method of generating light based on twolasers;

FIG. 23 is a flowchart of a method of generating light based on threelasers;

FIG. 24 is a top view of a projector optical design with dualillumination using LCOS SLMs;

FIG. 25 is a top view of a projector optical design with dualillumination using DMD SLMs;

FIG. 26 is a top view of a projector optical design with dualillumination using transmissive LCD SLMs;

FIG. 27 is a front view of a portrait-oriented SLM with two imageslocated one above the other;

FIG. 28 is a front view of a landscape-oriented SLM with two imageslocated one above the other;

FIG. 29 is a front view of a portrait-oriented SLM with two images farapart and located one above the other;

FIG. 30 is a front view of a landscape-oriented SLM with two imageslocated on the left and right of each other;

FIG. 31 is a front view of a landscape-oriented SLM with two imageslocated one diagonal to the other;

FIG. 32 is a front view of a landscape-oriented SLM with an anamorphicpattern of pixels;

FIG. 33 is a front view of a landscape-oriented SLM with a checkerboardpattern of pixels;

FIG. 34 is a top view of low etendue illumination compared to highetendue illumination;

FIG. 35 is a flow chart of a method of dual illumination;

FIG. 36 is a flowchart of an assembly method;

FIG. 37 is a flowchart of an assembly method with pre-alignment;

FIG. 38 is a flowchart of an assembly method with a chassis plate;

FIG. 39A is a top view of a base plate;

FIG. 39B is a side view of a base plate;

FIG. 40A is a side view of an optical module;

FIG. 40B is a bottom view of an optical module;

FIG. 41 is a side view of a mated base plate and optical module;

FIG. 42 is a side view of a mated base plate and optical module attachedto a chassis plate;

FIG. 43A is a top view of a chassis plate with multiple optical modulesattached;

FIG. 43B is a side view of a chassis plate with multiple optical modulesattached;

FIG. 44 is a flowchart of an assembly method with an alignment plate;

FIG. 45A is a top view of an alignment plate;

FIG. 45B is a side view of an alignment plate and a holding plate;

FIG. 46A is a schematic diagram of alignment error bars for the assemblymethod of FIG. 36;

FIG. 46B is a schematic diagram of alignment error bars for the assemblymethod of FIG. 37;

FIG. 47A is a top view of an optical assembly on a flat supportstructure;

FIG. 47B is a side view of an optical assembly on a flat supportstructure;

FIG. 48A is a top view of a compartmented support structure;

FIG. 48B is a side view of a compartmented support structure;

FIG. 48C is a top view of a compartmented support structure with afeedthrough hole in a different location;

FIG. 48D is a side view of a compartmented support structure with afeedthrough hole in a different location;

FIG. 49A is a top view of an optical assembly in a compartmented supportstructure;

FIG. 49B is a side view of an optical assembly in a compartmentedsupport structure;

FIG. 50A is a side view of a stacked compartmented support structure;

FIG. 50B is a top view of a stacked compartmented support structure;

FIG. 51 is a flowchart of an assembly method for a stacked compartmentedsupport structure assembled in parallel;

FIG. 52 is a flowchart of an assembly method for a stacked compartmentedsupport structure assembled in series;

FIG. 53 is a schematic view of an optical system with an opticalparametric oscillator;

FIG. 54 is a schematic view of a recirculating optical subsystem withfour relay lenses;

FIG. 55 is a schematic view of a recirculating optical subsystem withfour relay lenses showing additional details of the optical beams;

FIG. 56 is a schematic view of a non-rectilinear recirculating opticalsubsystem;

FIG. 57 is a flowchart of a method of generating light;

FIG. 58 is a top view of a gain module with flat mirrors;

FIG. 59 is a side view of a gain module with flat minors;

FIG. 60 is a top view of a gain module with retroreflective mirrors;

FIG. 61 is a flowchart of a method of using retroreflective mirrors;

FIG. 62 is a block diagram of an optical system with an optical tap;

FIG. 63A is a side view of an optical tap with two plates;

FIG. 63B is a side view of an optical tap with three plates;

FIG. 64 is a side view of an optical plate with an incident ray oflight;

FIG. 65 is a graph of reflection from an optical plate;

FIG. 66 is an expanded graph of reflection from an optical plate;

FIG. 67 is a block diagram of a color adjusting system with opticaltaps; and

FIG. 68 is flowchart of a method of optical tapping;

FIG. 69 is a flowchart of an optical coupler method;

FIG. 70 is a side view of an optical coupler with a center-drivendiffuser;

FIG. 71 is a side view of an optical coupler with an edge-drivendiffuser;

FIG. 72 is a side view of an optical coupler with a collimated outputbeam;

FIG. 73 is an isometric end view of an optical fiber bundle;

FIG. 74 is a side view of an optical coupler with an optical fiberbundle;

FIG. 75 is a side view of an optical coupler with a reflective beamcombiner;

FIG. 76 is a side view of an optical system with a fiber;

FIG. 77 is a cross sectional view of a flat-sided fiber;

FIG. 78 is a cross sectional view of a rectangular fiber with glasscladding;

FIG. 79 is a cross sectional view of a rectangular fiber with aircladding; and

FIG. 80 is a method of illuminating a digital projector using aflat-sided fiber.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of optical system 100 which has a number ofnovel parts that will be explained in the following description. Firstlaser light source 102 may include OPO 104, laser gain module 106, andoptical tap 108. Light is generated in OPO 104, passes through gainmodule 106, then through tap 108. The light then passes through firstcoupler 110 which includes flat-sided fiber 112, and then is processedby first SLMs 114. The light is then projected out of the opticalsystem.

Second laser light source 116 may optionally be included forstereoscopic optical systems. The light from second laser light source116 passes through second coupler 118 and then through first SLMs 114.The light is then projected out of the optical system. Alternatively,third laser light source 120 may optionally be included for stereoscopicoptical systems. The light from third laser light source 120 passesthrough third coupler 122 and then through second SLMs 124. The light isthen projected out of the optical system. Second laser light source 116and third laser light source 120 may also include OPOs, gain modules,and optical taps (not shown).

In one aspect of the optical system shown in FIG. 1, instead offiltering out damaging wavelengths of light from the light source as istypically done with conventional arc lamp sources, a light source may beused that does not generate the damaging wavelengths, thereby avoidingthe need to filter. By proper selection of the wavelength,specifications for color gamut and brightness may be met while utilizingall of the blue efficiently and maximizing the lifetime of liquidcrystal materials in the SLM. First laser light source 102, second laserlight source 116, or third laser light source 120 may be used to providelaser light without damaging wavelengths. Specific embodiments followbut are not meant to be limiting in any way.

High energy light may damage the internal parts of SLMs. Liquid crystalmaterials that are used in SLMs are particularly sensitive to shortwaveradiation. LCD and LCOS SLMs include liquid crystal materials. The lightwith highest energy photons is at the short end of the visible spectrumsuch as the short-wave end of the blue light region and UV light. In aconventional projector, these bands are filtered out so as to preventdegradation to the liquid crystal materials.

For a display system with a white light source, the brightness is inpart determined by the amount of blue light that can be filtered out ofthe original white light. Filtering the light source to remove damagingwavelengths of light at the shortwave end has the disadvantage oflowering the amount of blue light available to the overall opticalsystem. If the blue bandwidth is increased by moving the longwave end ofthe band up towards green, the separation between blue and green may belost. If the total system brightness is limited by the amount of bluelight, the other colors of light (green and red) must also be lowered ifthe amount of blue light is reduced in order to keep the desired whitepoint. Reducing all three colors then results in a large decrease intotal system brightness.

Another disadvantage of filtering out shortwave portions of the lightsource is that the color of the blue primary is changed if the shortwaveend of the blue band is eliminated. This may cause the projector's colorgamut to fall outside of the desired specification.

One type of display system projects an image onto an external screen.This is commonly known as a projector. Other types of display systemsmay have the screen contained internally. FIG. 2 shows the opticaldesign of a projector system that uses LCOS SLMs. Light source 200produces first beam segment 202 which is spread by first lens system 204to make second beam segment 206. Second beam segment 206 is homogenizedby mixing rod 208 to produce third beam segment 210. Third beam segment210 is spread by second lens system 212 to form fourth beam segment 214.Fourth beam segment 214 is collimated by third lens system 216 to makefifth beam segment 218. Fifth beam segment 218 partially reflects fromfirst DBS 220 to form sixth beam segment 250 and partially transmits toform seventh beam segment 222. Sixth beam segment 250 reflects frommirror 252 to form eighth beam segment 254. Eighth beam segment 254enters first PBS 256 and is reflected to form ninth beam segment 258.Ninth beam segment 258 is processed by first SLM 260 which rotates thepolarization of each pixel depending on the desired brightness of thepixel and reflects ninth beam segment 258 back along its input path toreenter first PBS 256. On a pixel-by-pixel basis, if the polarization isnot changed relative to the input beam, the light reflects inside firstPBS 256 to go back towards light source 200. If the polarization hasbeen changed relative to the input beam, some or all of the light(depending on how much the polarization has been changed) passes throughfirst PBS 256 to form tenth beam segment 262.

Seventh beam segment 222 partially reflects from second DBS 224 to formeleventh beam segment 240 and partially transmits to form twelfth beamsegment 226. Eleventh beam segment 240 enters second PBS 242 and isreflected to form thirteenth beam segment 244. Thirteenth beam segment244 is processed by second SLM 246 which rotates the polarization ofeach pixel depending on the desired brightness of the pixel and reflectsthirteenth beam segment 244 back along its input path to reenter secondPBS 242. On a pixel-by-pixel basis, if the polarization is not changedrelative to the input beam, the light reflects inside second PBS 242 togo back towards light source 200. If the polarization has been changedrelative to the input beam, some or all of the light (depending on howmuch the polarization has been changed) passes through second PBS 242 toform fourteenth beam segment 248.

Twelfth beam segment 226 enters third PBS 228 and is reflected to formfifteenth beam segment 230. Fifteenth beam segment 230 is processed bythird SLM 234 which rotates the polarization of each pixel depending onthe desired brightness of the pixel and reflects fifteenth beam segment230 back along its input path to reenter third PBS 228. On apixel-by-pixel basis, if the polarization is not changed relative to theinput beam, the light reflects inside third PBS 228 to go back towardslight source 200. If the polarization has been changed relative to theinput beam, some or all of the light (depending on how much thepolarization has been changed) passes through third PBS 228 to formsixteenth beam segment 236.

Beam combiner 238 combines tenth beam segment 262, fourteenth beamsegment 248, and sixteenth beam segment 236 to form seventeenth beamsegment 264. Seventeenth beam segment 264 passes through fourth lenssystem 266 to form eighteenth beam segment 268 which passes outside ofthe projector to make a viewable image on a projection screen (notshown).

Beam combiner 238 may be an X-prism. First lens system 204, second lenssystem 212, third lens system 216, and fourth lens system 266 may beformed from a single lens or any number of lenses that guide the lightbeams into the desired positions. First lens system 204 may be anoptical fiber or fiber bundle. The sizes of components and distancesbetween components are not shown to scale in FIG. 2. Some opticalcomponents may be positioned against other optical components so thatthere is no gap between the components. Auxiliary optical componentssuch as polarizers, relay lenses, skew ray plates, polarization rotationplates, and trim filters are not shown in FIG. 2. The three SLMs shownin FIG. 2 may be each assigned to a primary color so that one is red,one is green, and one is blue. Light source 200 may output polarizedlight. Mixing rod 208 may be replaced by a fly's eye lens or otherhomogenization component in which case first lens system 204, secondlens system 212, and third lens system 216 may take a different form inorder to guide the light properly through the homogenization component.

FIG. 3 shows the optical design of a projector system that usestransmissive LCD SLMs. Light source 300 produces first beam segment 302which is spread by first lens system 304 to make second beam segment306. Second beam segment 306 is homogenized by mixing rod 308 to producethird beam segment 310. Third beam segment 310 is spread by second lenssystem 312 to form fourth beam segment 314. Fourth beam segment 314 iscollimated by third lens system 316 to form fifth beam segment 318.Fifth beam segment 318 partially reflects from first DBS 320 to formsixth beam segment 342 and partially transmits to form seventh beamsegment 322. Sixth beam segment 342 reflects from first mirror 344 toform eighth beam segment 346. Eighth beam segment 346 is processed byfirst SLM 348 which rotates the polarization of each pixel depending onthe desired brightness of the pixel. On a pixel-by-pixel basis,depending on the amount of polarization rotation, the light istransmitted or absorbed to a varying degree by a polarizer (not shown)to form ninth beam segment 350.

Seventh beam segment 322 partially reflects from second DBS 324 to formtenth beam segment 352 and partially transmits to form eleventh beamsegment 326. Tenth beam segment 352 is processed by second SLM 354 whichrotates the polarization of each pixel depending on the desiredbrightness of the pixel. On a pixel-by-pixel basis, depending on theamount of polarization rotation, the light is transmitted or absorbed toa varying degree by a polarizer (not shown) to form twelfth beam segment356.

Eleventh beam segment 326 reflects from second mirror 328 to formthirteenth beam segment 330. Thirteenth beam segment 330 reflects fromthird minor 332 to form fourteenth beam segment 334. Fourteenth beamsegment 334 is processed by third SLM 336 which rotates the polarizationof each pixel depending on the desired brightness of the pixel. On apixel-by-pixel basis, depending on the amount of polarization rotation,the light is transmitted or absorbed to a varying degree by a polarizer(not shown) to form fifteenth beam segment 338.

Beam combiner 340 combines ninth beam segment 350, twelfth beam segment356, and fifteenth beam segment 338 to form sixteenth beam segment 358.Sixteenth beam segment 358 passes through fourth lens system 360 to formseventeenth beam segment 362 which passes outside of the projector tomake a viewable image on a projection screen (not shown).

Beam combiner 340 may be an X-prism. First lens system 304, second lenssystem 312, third lens system 316, and fourth lens system 360 may beformed from a single lens or any number of lenses that guide the lightbeams into the desired positions. First lens system 304 may be anoptical fiber or fiber bundle. The sizes of components and distancesbetween components are not shown to scale in FIG. 3. Some opticalcomponents may be positioned against other optical components so thatthere is no gap between the components. Auxiliary optical componentssuch as polarizers, relay lenses, skew ray plates, polarization rotationplates, and trim filters are not shown in FIG. 3. The three SLMs shownin FIG. 3 may be each assigned to a primary color so that one is red,one is green, and one is blue. Light source 300 may output polarizedlight. Mixing rod 308 may be replaced by a fly's eye lens or otherhomogenization component in which case first lens system 304, secondlens system 312, and third lens system 316 may take a different form inorder to guide the light properly through the homogenization component.

FIG. 4 shows a graph of the spectrum of a xenon lamp. Xenon lamps are atype of high-intensity-discharge lamp based on the optical emission ofxenon gas and are commonly used for illumination of large-venue digitalprojectors. The x-axis of FIG. 4 represents wavelength in nanometers andthe vertical axis represents normalized intensity. Curve 400 shows thespectrum of a typical short-arc xenon lamp of the type used in digitalprojectors. There is a substantial amount of the spectrum in wavelengthsbelow 430 nm that may be harmful to liquid crystal materials in LCD orLCOS SLMs.

FIG. 5 shows a graph of the shortwave spectrum of a xenon lamp. Thex-axis of FIG. 5 represents wavelength in nanometers and the verticalaxis represents normalized intensity. Curve 500 is obtained by filteringout the wavelengths longer than 500 nm in curve 400 of FIG. 4. Curve 500shows the spectrum of the blue channel in a xenon-lamp-based digitalprojector if the entire shortwave spectrum is used to make the blueprimary color. In this case, the blue channel of the digital projectorwould be subject to damage from wavelengths shorter than 430 nm, so thespectrum shown in FIG. 5 may be considered unsafe with respect to thelifetime of LCD or LCOS SLMs.

FIG. 6 shows a graph of the portion of the blue spectrum of a xenon lampthat is safe for liquid crystal materials. The x-axis of FIG. 6represents wavelength in nanometers and the vertical axis representsnormalized intensity. Curve 600 is obtained by filtering out thewavelengths shorter than 430 nm in curve 500 of FIG. 5. Curve 600 showsthe spectrum of the blue channel in a xenon-based digital projector ifonly the safe wavelengths are used to make the blue primary color. Thecase shown in FIG. 6 would result in an improved SLM lifetime for theblue channel as compared to the case shown in FIG. 5.

FIG. 7A shows a graph of the spectrums of two blue lasers withwavelengths of 450 and 455 nm. The x-axis of FIG. 7A representswavelength in nanometers and the vertical axis represents normalizedintensity. The blue lasers have each a single wavelength with a narrowbandwidth, so the two spectrums appear as first vertical line 700 at 450nm and second vertical line 702 at 455 nm. In these two examples, thereare no wavelengths less than 430 nm, so there would be improved SLMlifetime for the blue channel as compared to the case shown in FIG. 5.For the purposes of this disclosure, narrow band is considered to be afull-width-half-maximum of less than 1 nm and wide band is considered tobe a full-width-half-maximum of more than 1 nm.

FIG. 7B shows a graph of the spectrum of a wide-band blue laser thatcovers the range of wavelengths between 450 and 455 nm. The centerwavelength is 452.5 nm. The x-axis of FIG. 7B represents wavelength innanometers and the vertical axis represents normalized intensity. Inthis example, there are no wavelengths less than 430 nm, so there wouldbe improved SLM lifetime for the blue channel as compared to the caseshown in FIG. 4. The wide-band laser spectrum is shown as square waveform 704 in FIG. 7B, but may have other shapes such as a Gaussian form,and it may have peaks and dips at specific wavelengths. In the case ofcomplex shapes, the center wavelength is generally defined as thewavelength midway between the two half-maximum points.

FIG. 8 shows a color chart of two xenon-projector color gamuts comparedto the DCI standard. The x and y axes of FIG. 8 represent the x and ycoordinates of the CIE 1931 color space. Each color gamut is shown as atriangle formed by red, green, and blue primary colors that form thecorners of the triangle. All other colors of the projector are made bymixing various amounts of the three primaries to form the colors insidethe gamut triangle. Colors outside the gamut triangle cannot be made bythe projector. First triangle 800 shows the DCI standard which iscommonly accepted for digital cinema in large venues such as movietheaters and museums. Second triangle 802 shows a projector gamut withthe unsafe spectrum of a xenon lamp, as shown in FIG. 5, forming theblue primary. Third triangle 804 shows a projector gamut with the safespectrum of a xenon lamp, as shown in FIG. 6, forming the blue primary.The green and red primaries of second triangle 802 and third triangle804 are chosen to be the same as the green and red primaries of the DCIstandard for the purposes of this example. It is desirable to chooseprojector primary colors such that the projector gamut includes theentire gamut of the applicable standard so that all of the colors in thestandard may be formed by the projector. Note that second triangle 802covers more of first triangle 800 than does third triangle 804. Thisshows that the safe spectrum's color gamut is significantly worse thanthe unsafe spectrum's color gamut. In the blue and purple regions of thecolor chart, the safe spectrum is not able to reproduce some of thecolors required by the DCI standard that are available with the unsafespectrum.

FIG. 9 shows a color chart of two laser-projector color gamuts comparedto the DCI standard. The x and y axes of FIG. 9 represent the x and ycoordinates of the CIE 1931 color space. First triangle 900 shows theDCI standard. Second triangle 902 shows a projector gamut with a bluelaser at 450 nm (as shown by first vertical line 700 in FIG. 7A) formingthe blue primary. Third triangle 904 shows a projector gamut with a bluelaser at 455 nm (as shown by second vertical line 702 in FIG. 7A)forming the blue primary. The green primaries of the laser projectorsare at 523.5 nm and the red primaries are at 625 nm for the 450 nm blueprimary and 615 nm for the 455 nm blue primary. These green and redwavelengths are the result of using OPO laser light sources as describedin U.S. Pat. No. 5,740,190, the complete disclosure of which isincorporated herein by reference. Other laser wavelengths may beutilized with similar results. Second triangle 902 and third triangle904 each include first triangle 900 showing that both examples of laserprojectors are able to form all the colors required by the DCI standard.

FIG. 10 shows a color chart of a laser-projector color gamut compared tothe Rec. 709 standard. The Rec. 709 standard is commonly accepted fordigital projection of high definition content in homes. The x and y axesof FIG. 10 represent the x and y coordinates of the CIE 1931 colorspace. First triangle 1000 shows the Rec. 709 standard. Second triangle1002 shows a projector gamut with a blue laser at 455 nm (as shown bysecond vertical line 702 in FIG. 7A) forming the blue primary. The greenprimaries of the laser projector is at 523.5 nm and the red primary isat 615 nm. Other laser wavelengths may be utilized with similar results.Second triangle 1002 includes first triangle 1000 showing that theexample laser projector is able to form all the colors required by theRec. 709 standard.

FIG. 11 shows a graph of the spectrum of a UHP lamp. UHP lamps are atype of short-arc high-intensity-discharge lamp based on the opticalemission of mercury vapor and are commonly used for illumination of homedigital projectors. The x-axis of FIG. 11 represents wavelength innanometers and the vertical axis represents normalized intensity. Curve1100 shows the spectrum of a typical UHP lamp of the type used in homedigital projectors. As with the spectrum of the xenon lamp shown in FIG.4, FIG. 11 shows that a UHP lamp also has a substantial amount of thespectrum in wavelengths below 430 nm that may be harmful to liquidcrystal materials in LCD or LCOS SLMs.

SLMs may be formed from a variety of light valve technologies. Some arebased on polarized light such as LCD and LCOS technologies whereas somedo not depend on polarized light such as DMDs. DMD SLMs are usually notas sensitive to shortwave optical radiation as LCD and LCOS SLMs, buteven DMD SLMs may be adversely affected if the intensity of theshortwave radiation is high enough. Other types of light valves are lesscommonly used and depending on their materials, may or may not beaffected by shortwave optical radiation. Liquid crystal materials areused to rotate the polarization of light in LCD and LCOS SLMs. Forexample, if polarized light is incident on a liquid crystal pixel, bycontrolling the voltage across the liquid crystal material, thepolarization state of light passing through the pixel may be variablychanged depending on the voltage. A polarizer or PBS acts as a filter onthe exit side of the light valve to allow through only light that is ofthe desired polarization. A dark pixel is formed when the polarizer orPBS does not allow transmission out of the projector and a bright pixelis formed when the polarizer or PBS does allow transmission out of theprojector. Any intermediate state of polarization can make anintermediate brightness pixel. The primary optical characteristic of theSLM (and associated optical elements such as the polarizer and PBS) iscontrast between the darkest possible state and the brightest possiblestate. Other optical characteristics include switching speed and maximumpossible transmission. Light with wavelengths shorter than 430 nm maydegrade the optical characteristics of the SLM and its associatedoptical elements. The degradation may take the form of decreasedcontrast, decreased light transmission, increased switching speed, or adegradation of any other optical characteristic necessary for the properfunctioning of the SLM. If only light above 430 nm is used, the opticalcharacteristics of the SLM will be preserved leading to an improvedlifetime for the optical system.

There are two types of LCOS SLMs: organic and inorganic. Organic LCOSSLMs use an organic material such as a polyimide plastic to align theliquid crystal layer, whereas inorganic LCOS SLMs use an inorganicalignment material such as SiO₂. Although LCOS SLMs with organicalignment layers are more easily damaged by shortwave optical radiation,inorganic LCOS SLMs are also damaged given enough time and intensity.

In order to achieve the desired brightness, a light source of sufficientoutput should be used in a projection system. For a given light sourcetechnology, light output tends to scale with light-source powerconsumption (wattage), but there are limits on the maximum wattage lightsource that can be efficiently utilized in a projection system. Forexample, arc lamps for cinema projectors are typically limited to about7 kilowatts for DMD-based projectors because at higher powers, the arcbecomes too long and the etendue of the light source exceeds the etendueof the DMD projection system. This prevents efficient use of the lightgenerated in the light source. For LCOS projectors, a 7 kilowatt lampmight be accepted by the etendue of a given LCOS projection system, butthe limiting factor may instead be the amount of damaging shortwaveoptical radiation that is generated by the arc lamp. At even higherpowers, the etendue of the lamp may still become the limiting factor. Anunfiltered laser light system has the advantage that an arbitrarilylarge power can be used without the limitations of damage by shortwaveoptical radiation.

The 430 nm definition of shortwave optical radiation is an approximationbecause the wavelengths of light that can cause damage depend on theexact construction of the SLM, the composition of the liquid crystalmaterial, and the material of the associated optical components such aspolarizers or PBS s among other factors. Wavelengths below approximately200 nm are emitted by arc lamp sources, but are strongly absorbed by theair, so they are not a factor in causing damage to SLMs. The mostdamaging rays are generally in the range of 200 nm to 430 nm which isthe region considered shortwave optical radiation. The range ofwavelengths from 200 nm to 380 nm is considered UV light and the rangeof wavelengths from 380 nm to 430 nm is considered shortwave blue light.The range of wavelengths between 430 nm and 500 nm is consideredlongwave blue light.

In order to eliminate damaging wavelengths of light from an arc lampillumination source, projectors usually use optical filters. These aretypically shortwave cut-off filters that operate on principles ofmultilayer interference or absorption or both. These filter designs havea gradual cut-off rather than an infinitely sharp cut-off, so for thepurposes of this description, the cutoff wavelength may be considered asthe wavelength where 10% of the light is transmitted. Even in thetransmission band at longer wavelengths than the cut-off wavelength,there is reduced transmission due to reflections or absorptions in thefilter at the longer wavelengths. These losses contribute to thedesirability of using illumination systems that do not have a cut-offfilter.

A stereoscopic projection system may be formed by using the principle ofspectral selection to separate left and right images as described inU.S. Pat. No. 6,283,597. In the spectral selection method, first primarycolors of red, green, and blue are passed to the left eye, and secondprimary colors of red, green, and blue are passed to the right eye. Thefirst group of primaries and the second group of primaries are distinctso that there is little or no overlap between the first and secondgroups. To achieve sufficient separation between the two blue primaries,the two wavelengths may be approximately 10 to 20 nm apart. The firstblue primary wavelength may be in the range of 450 nm to 460 nm and thesecond blue primary wavelength may be in the range of 430 nm to 445 nm.With an OPO light source, an effective first blue primary wavelength maybe 445 nm and an effective second blue primary wavelength may be 440 nm.Unfiltered illumination sources have a particular advantage in the caseof spectrally selective stereoscopic projection systems because thefilters used to cut out the short wave blue light typically decrease theamount of light available for the second (shorter wave) blue primary.

Blue light in the safe region may be generated by methods other than anOPO. Other types of blue lasers may be used instead or other types ofblue light sources that generate blue light in the safe region such assemiconductor light sources. One type of semiconductor light source is ablue LED. Individual LEDs may be arrayed in bars or other configurationsto produce powerful sources of blue light. Also, diode-pumpedsolid-state lasers may be used to generate blue light with non-linearoptical elements. The green wavelengths may be likewise be produced bylasers or LEDs centered at various wavelengths such as 520 nm, 532 nm,or 540 nm, and the red wavelengths may be produced by lasers or LEDscentered at various wavelengths such as 620 nm, 630 nm, or 640 nm.

In another aspect of the optical system shown in FIG. 1, apolarization-switching light source is used to form stereoscopic imagesby transmission through a polarization-preserving projection device.First laser light source 102 may be a polarization switching lightsource. First coupler 110 and first SLMs 114 may bepolarization-preserving parts of a polarization-preserving projectiondevice. Specific embodiments are described in the following paragraphsbut are not meant to be limiting in any way.

First laser light source 102, second laser light source 116, or thirdlaser light source 120 may be used to provide polarization-switchedlight. Polarized light may be used to project stereoscopic images whenthere are two orthogonal states of polarization. Orthogonal polarizationstates mean that two different orientation or types of polarizingfilters are able to fully and distinctly separate the two polarizationstates without overlap. For example, linearly polarized light with anelectric field vector oscillating in the horizontal direction isorthogonal to linearly polarized light with the electric field vectoroscillating in the vertical direction. Similarly, linearly polarizedlight with an electric field vector oscillating in the −45 degreedirection is orthogonal to linearly polarized light with the electricfield vector oscillating in the +45 degree direction. Also, circularlypolarized light with an electric field vector rotating in the clockwise(right-hand) direction is orthogonal to circularly polarized light withan electric field vector rotating in the counter-clockwise (left-hand)direction. If two states of non-orthogonal polarized light are used forstereoscopic viewing, the left eye image will leak into the right eyeand vice versa to make ghosting artifacts which detract from the qualityof the stereoscopic viewing experience.

Circularly polarized light is commonly used for stereoscopic projectionbecause viewer head tilt does not significantly change the viewingexperience. In contrast, when using linearly polarized light, severeghosting artifacts will appear if the viewer's head is tilted too far.Also if the projector alone produces linearly polarized light, anexternal device may be placed in front of the projector to rapidlychange the polarization between left-hand and right-hand circularpolarization states. The external device may be anelectrically-controlled liquid-crystal polarization rotator which isdriven in synchronization with the projection of left-eye images andright-eye images.

By alternating the polarization states of left-eye images and right-eyeimages, stereoscopic content may be displayed and viewed. By rapidlyalternating the polarization and images at faster than theflicker-fusion frequency, the appearance of moving images may beobtained.

Polarization preserving optical elements are used to transmit or reflectlight without changing its polarization state. If two orthogonalpolarization states are transmitted and reflected multiple times by theoptical elements in a polarization preserving optical system, the twoorthogonal polarization states are maintained in the original twoorthogonal polarization states. Birefringence is the property of opticalelements that describes the case where different directions havedifferent indices of refraction. Birefringence generally leads tochanges in polarization state, so low birefringence is usually desirablewhen designing polarization preserving optical systems.

FIG. 12 shows a stereoscopic display system. Light source 1200illuminates projector 1202. Projector 1202 projects an image with lens1204. Projector 1202 and lens 1204 illuminate polarizing filter 1206which polarizes the light into one polarization state. After polarizingfilter 1206, the light passes into polarization switch 1208 whichactively switches the light into two orthogonal polarization states insynchronization with projector 1202 displaying left and right eyeimages. The light then passes to polarization-preserving screen 1210 toform projected image 1212 on polarization-preserving screen 1210. Inthis system, the images for one eye are displayed with one orthogonalpolarization state while the images for the other eye are displayed withthe other orthogonal polarization state.

FIG. 13 shows a stereoscopic display system with apolarization-switching light source. Polarization-switching light source1300 illuminates polarization-preserving projector 1302.Polarization-preserving projector 1302 projects an image with lens 1304.Polarization-switching light source 1300 switches the light into twoorthogonal polarization states in synchronization withpolarization-preserving projector 1302 displaying left and right eyeimages. Polarization-preserving projector 1302 andpolarization-preserving lens 1304 illuminate polarization-preservingscreen 1310 to form projected image 1312 on polarization-preservingscreen 1310. As in the system of FIG. 12, the images for one eye aredisplayed with one orthogonal polarization state while the images forthe other eye are displayed with the other orthogonal polarizationstate.

FIG. 14 shows the details of a polarization-preserving projector design.Light from polarization-switching light source 1420 enters lens system1400. Lens system 1400 passes the light to TIR prism 1422 which consistsof first TIR subprism 1402 and second TIR subprism 1412. The lightenters first TIR subprism 1402 and reflects off the interface betweenfirst TIR subprism 1402 and second TIR subprism 1412. Then the lightexits first TIR subprism 1402 and enters Philips prism 1424 whichconsists of first Philips subprism 1404, second Philips subprism 1406,and third Philips subprism 1408. The light enters first Philips subprism1404 and passes to the interface between first Philips subprism 1404 andsecond Philips subprism 1406. At the interface between first Philipssubprism 1404 and second Philips subprism 1406, the blue wavelengthregion of the light is reflected and the green and red wavelengthregions of the light are transmitted. The blue light reflects off thesurface of first Philips subprism 1404, then reflects from bluepolarization-preserving SLM 1416, then reflects off the surface of firstPhilips subprism 1404, then reflects off the interface between firstPhilips subprism 1404 and second Philips subprism 1406 to rejoin themain beam to exit from Philips prism 1424. The green light passes intosecond Philips subprism 1406, then into third Philips subprism 1408,then reflects from green polarization-preserving SLM 1410, then rejoinsthe main beam to exit from Philips prism 1424. The red light reflectsfrom the interface between second Philips subprism 1406 and thirdPhilips subprism 1408, reflects off the surface of second Philipssubprism 1406, then reflects from red polarization-preserving SLM 1418,then reflects from the interface of first Philips subprism 1404 andsecond Philips subprism 1406, then reflects from the interface of secondPhilips subprism 1406 and third Philips subprism 1408 to join the mainbeam and to exit from Philips prism 1424. The beam with all three colorsmodulated by polarization-preserving SLMs 1410, 1416, and 1418 passesagain through TIR prism 1422 and then passes throughpolarization-preserving lens 1414 to form a projected digital image.

FIG. 15 shows a polarization-switching light source. Linearly polarizedlight source 1500 forms linearly polarized light beam 1502. Linearlypolarized light beam 1502 passes through quarter wave plate 1504 whichproduces right-hand circularly polarized light beam 1506. Right-handcircularly polarized light beam 1506 passes through polarization switch1518 which consists of dummy plate 1508, half wave plate 1512, rotor1514, and motor 1516. Dummy plate 1508 and half wave plate 1512 formrotating disk 1520 which spins around rotor 1514 and is powered by motor1516. Right-hand circularly polarized light beam 1506 passes alternatelythrough dummy plate 1508 and half wave plate 1512 as they spin aroundrotor 1514. When right-hand circularly polarized light beam 1506 passesthrough dummy plate 1508, there is no effect on the polarization andbeam 1510 is still right-hand circularly polarized. When right-handcircularly polarized light beam 1506 passes through plate 1512, thepolarization of beam 1510 is changed to become left-hand circularlypolarized. Polarization switch 1518 is synchronized with a projector toproduce left-eye images polarized with one circular polarization state(e.g. right-hand) and right eye images with the orthogonal circularpolarization state (e.g. left-hand). Alternately, quarter wave plate1504 may be arranged to produce left-hand circularly polarized light inwhich case half wave plate 1512 changes the polarization to right-handcircularly polarized light.

FIG. 16 shows a front view of rotating disk 1520 in polarization switch1518. Disk 1520 rotates clockwise as shown by arrow 1602. Right-handcircularly polarized light beam 1506 passes through disk 1520 at theposition shown by arrow 1600. Alternately, the disk may rotatecounterclockwise.

FIG. 17 shows a method of stereoscopic projection. In step 1700,polarization-switched light is generated. In step 1702, the polarizationswitched light is modulated to make a left-eye digital image and aright-eye digital image. In step 1704, the left-eye digital image andthe right-eye digital image are projected for viewing. The modulation instep 1702 may be in synchronization with the polarization of thepolarization-switched light so that the image for the left eye ispolarized in one state, and the image for the right eye is polarized inthe orthogonal state.

Polarization-switching light sources may be constructed by polarizing anaturally unpolarized source of light such as an arc lamp. In this case,the polarizer may be an absorptive polarizer such as polarizing filmthat absorbs the unwanted polarization of light. A more efficient systemmakes use of polarization recovery to repolarize the unwantedpolarization state so that there is more light in the desiredpolarization state. Another method is to start with an inherentlypolarized light source such as a polarized laser that may be a solidstate laser, diode pumped solid state laser, gas laser, or OPO.

A polarization switch may be used to actively change the polarizationstate of the polarization-switching light source. A polarization switchmay be mechanical such as the one shown in FIG. 15, or it may be anelectro-optical or magneto-optical device such as a polarization cellbased on the Pockels effect, Faraday effect, Kerr effect, or any otherpolarization-controlling optical element. The polarization switch may bebuilt into the light source or it may be a separate element immediatelyafter the light source which operates on light emitted by the lightsource in which case the light source and polarization switch togetherare considered to be a polarization-switching light source. Whenutilized for stereoscopic projection, the polarization switch should notspend significant time switching between states because the time betweenstates may contribute to ghosting.

Wave plates may be also used to change polarization states of light.Achromatic wave plates make the same change in polarization across allwavelengths in a certain design region. When held at the properrotational orientation relative to the beam direction of propagation,quarter wave plates make a 90 degree phase difference in the horizontaland vertical electric field vectors such that linearly polarized lightis converted to circularly polarized light and vice versa. Half waveplates make a 180 degree phase difference such that linearly orcircularly polarized light is converted to the orthogonal polarizationstate. Wave plates may be made of birefringent crystals cut as aspecific orientation, or they may be made from birefringent plastic filmwith specific retardation in each axis. Achromatic wave plates may bemade from a stack of multiple layers of plastic film at specificorientations such as those manufactured by ColorLink Inc. (Boulder,Colo.).

A polarization-preserving projector may be constructed by using opticalcomponents that are themselves polarization preserving. If all thelenses, prisms, SLMs, minors and other optical components of theprojector are polarization preserving, the overall projector will bepolarization preserving. Optical components are polarization preservingif they cause an acceptably low level of polarization changes for twoorthogonal polarization states. Optical effects which can causepolarization changes include scattering, retardation, and polarizationsplitting. Scattering is an inherently depolarizing process. Retardationmay occur from randomly distributed birefringence regions in plasticoptical elements. Polarization splitting is caused at optical surfacesby differences in reflected or transmitted intensities of light that areof two different polarization states. Optical surfaces with a very hightransmission or reflection throughout the wavelengths of operationgenerally have minimal polarization splitting. Certain AR coatings andall total internal reflection surfaces have minimal polarizationsplitting as long as the wavelengths and angles fall within theirdesigned range of operation. Also, most coatings and materials haveminimal polarization splitting when the AOI is low. The AOI for a ray oflight incident on a surface is defined as the angle between the incidentray of light and the perpendicular to the surface. If the AOI is zerodegrees, mirrors and transmissive surfaces have zero polarizationsplitting. If the AOI is 5 degrees, aluminum mirrors have approximately0.1% polarization splitting and typical AR coatings have approximately0.01%. If the AOI is 10 degrees, aluminum mirrors have approximately0.2% polarization splitting and typical AR coatings have approximately0.05% in the photopically significant middle of the visible region oflight which extends from approximately 500 nm to approximately 600 nm.

Optical elements constructed of high quality optical glass are generallyinherently polarization preserving in the bulk of the material becausethe index of refraction is uniform throughout. Plastic optical elements,on the other hand, sometimes have retardation which leads todepolarizing or non-uniform polarization if the index of refractionvaries throughout the plastic material.

Polarization preserving SLMs may be designed using DMDs such as thoseavailable from Texas Instruments (Dallas, Texas). Since the mirrors of aDMD are coated with a highly-reflecting material such as aluminum, thepolarization splitting is low at small angles of operation which aretypically 5 to 10 degrees AOI. If lower polarization splitting isdesired, higher reflection coatings or other coatings with reducedpolarization splitting may be used on the minors. In addition, ARcoatings on the DMD window may help reduce polarization splitting of theDMD.

Glass TIR prisms may be designed to be polarization preserving. Theinterface between the two subprisms of a TIR prism has an air gap withAR coatings on both of the surfaces that form the air gap. In theexample of FIG. 14, the AOI at the point where the beam passes throughthe air gap and AR coatings is 36 degrees. The transmission through theAR-coated surfaces of the TIR prism may have low polarization splittingif the AR-coating is designed to minimize polarization splitting.Between 500 and 600 nm, the polarization splitting may be less than0.2%.

Philips prisms may be used for color splitting and recombining asdescribed in detail in U.S. Pat. No. 3,659,918 the complete disclosureof which is incorporated herein by reference. Philips prisms with lowpolarization splitting may be constructed as described in “Design ofNon-Polarizing Color Splitting Filters used for Projection DisplaySystem” by W. Chen et al., Displays, Elsevier, 2005, the complete textof which is incorporated herein by reference. The Philips prisms used byChen have coatings designed with standard techniques of optical thinfilm design so that polarization splitting is reduced to near zero forall wavelengths of operation.

Polarization-preserving front-projection screens are commerciallyavailable with matte metallic coatings that diffusely reflect lightwhile maintaining the polarization state with low depolarization. Thesescreens are commonly used with polarization-based stereoscopicprojection systems. Polarization-preserving rear projection screens arealso available for use in rear projection systems.

Instead of using a projector that is polarization preserving, aprojector that makes a known change in polarization states may be usedas long as optical elements are included that perform compensatingpolarization adjustments after the light passes through the projector orinternally in the projector so that the left and right eye images arestill orthogonal. ColorLink filters may be helpful compensation elementsfor this purpose because they may be designed to change the polarizationof different colors by different amounts. For example, if thepolarization state of a specific color is rotated because ofpolarization splitting in the Philips prism, the polarization of thatcolor can be corrected by rotating that color back to the desiredpolarization state by adding a ColorLink filter at the output of theprojector.

An advantage of using a polarized-laser light source with apolarization-preserving projector is the high efficiency compared tosystems that start with an unpolarized light source. With DMD lightvalves, typical polarized projection systems lose at least 50% of thelight when using an unpolarized light source. Even with polarizationrecovery, 20% of the light is generally lost when converting fromunpolarized to polarized light.

As opposed to SLM-based projectors, scanning projectors do not use anSLM. Instead they have a spot or line of light that is scanned over thearea of the screen to form an image. Scanning projectors are typicallybased on laser light sources because the high collimation of the laserbeam allows the projector to form a small spot at a distance. Whencompared to scanning projectors, SLM-based projectors typically haveadvantages in construction simplicity, alignment stability, and safetydue to lower peak beam intensity.

In another aspect of the optical system shown in FIG. 1, a stereoscopicprojection system separates left and right images by using spectralselection, for example as described in U.S. Pat. No. 6,283,597. In thespectral selection method, first wavelength bands of red, green, andblue are passed to the left eye, and second wavelength bands of red,green, and blue are passed to the right eye. The first bands and secondbands are distinct so that there is little or no overlap between thefirst and second bands. First laser light source 102 may be used toprovide the first bands of red, green, and blue light and second laserlight source 116 or third laser light source 120 may be used to providethe second bands of red, green, and blue light. Specific embodiments aredescribed in the following paragraphs but are not meant to be limitingin any way.

FIG. 18 shows the general layout of a stereoscopic image projectionsystem. Projector 1800 emits light beam 1804 from lens 1802. Light beam1804 impinges on screen 1806 and reflects light beam 1808 through filterglasses 1810 to the eyes 1812 of viewer 1814. Projector 1800 may formdistinct images for each eye 1812 by time sequencing different imagesfor each eye 1812. Alternately, projector 1800 may be two separateprojectors, one forming the image for the left eye and one forming theimage for the right eye.

FIG. 19 shows an example of the operation of projector 1800 whichincludes light system 1918 and projection engine 1916. Light system 1918emits first light beam segment 1900 into projection engine 1916. Firstlight beam segment 1900 enters mixing rod 1902 which forms second lightbeam segment 1904. Second light beam segment 1904 enters lens 1906 whichforms third light beam segment 1920. Third light beam segment 1920enters splitting prism 1908 which forms fifth light beam segment 1922.Fifth light beam segment 1922 enters color prism 1910 and is separatedinto three colors which then impinge on light valves 1912. Each lightvalve 1912 forms an image in a distinct color and then the imaged lightbeams are combined by color prism 1910 and sent back along the path offifth light beam path segment 1922 into splitting prism 1908. Splittingprism 1908 outputs the imaged light beam as sixth light beam segment1914 to form an image on screen 1806. Splitting prism 1908 may be atotal TIR prism and color prism 1910 may be a Philips prism.

FIG. 20 shows one embodiment of light system 1918 which is based on twolasers. Nd:YLF laser 2000 generates light at 1047 nm which enters Nd:YLFgain module 2002, is optically amplified by gain module 2002, thenenters SHG unit 2004 where it is converted to green light at 523.5 nm.The green light is rapidly switched by left/right switch 2006 so that itpasses to either OPO 2008 or OPO 2010. A fraction of the 523.5 nm lightin OPO 2008 is converted to 910 nm and a fraction is converted to 1230nm. The 910 nm light exits OPO 2008 and is converted by SHG unit 2036into 455 nm blue light. The 1230 nm light exits OPO 2008 and isconverted by SHG unit 2038 into 615 nm red light. The 532.5 nm greenlight that is not lost in OPO 2008 or converted into blue or red lightby OPO 2008, exits OPO 2008. The left/right switch 2006 may besynchronized with the display of left and right images to use the greenlaser light efficiently.

A fraction of the 523.5 nm light in OPO 2010 is converted to 880 nm anda fraction is converted to 1288 nm. The 880 nm light exits OPO 2010 andis converted by SHG unit 2042 into 440 nm blue light. The 1288 nm lightexits OPO 2010 and is converted by SHG unit 2044 into 644 nm red light.The 532.5 nm green light that exits OPO 2010 goes into beam dump 2012.

Nd:YAP laser 2028 generates light at 1079.5 nm which enters Nd:YAP gainmodule 2030, is optically amplified by gain module 2030, then enters SHGunit 2040 where it is converted to green light at 539.7 nm.

The 455 nm blue light from OPO 2008 and the 440 nm blue light from OPO2010 pass through blue filter 2014 and form light beam 2020. The 523.5nm green light from OPO 2008 and the 539.7 nm green light from SHG unit2040 pass through green filter 2016 and form light beam 2022. The 615 nmred light from OPO 2008 and the 644 nm red light from OPO 2010 passthrough red filter 2018 and form light beam 2024. Light beams 2020,2022, and 2024 are combined together to form light beam 1900.

OPO 2008, SHG unit 2036, and SHG unit 2038 form light generation unit2032 which is used to form images for the left eye. Nd:YAP laser 2028,ND:YAP gain module 2030, SHG unit 2040, OPO 2010, SHG unit 2042, beamdump 2012, and SHG unit 2044 form light generation unit 2034 which isused to form images for the right eye. Alternately, light generationunit 2032 may be used for the right eye and light generation unit 2034may be used for the left eye.

Light system 1918 as described in FIG. 20 may produce at least 50 wattsof optical power including all six colors of output in light beams 2020,2022, and 2024. If the total optical power output is 100 watts, Nd:YLFlaser 2000 may produce about 2 watts of 1047 nm light and Nd:YLF gainmodule 2002 may produce about 300 watts of 1047 nm light. Nd:YAP laser2028 may produce about 2 watts of 1079.5 nm light and Nd:YAP gain module2030 may produce about 60 watts of 1079.5 nm light.

FIG. 21 shows another embodiment of light system 1918 which is based onthree lasers. Nd:YLF laser 2100 generates light at 1047 nm which entersNd:YLF gain module 2102, is optically amplified by gain module 2102,then enters SHG unit 2104 where it is converted to green light at 523.5nm and enters OPO 2108. A fraction of the 523.5 nm light in OPO 2108 isconverted to 910 nm and a fraction is converted to 1230 nm. The 910 nmlight exits OPO 2108 and is converted by SHG unit 2136 into 455 nm bluelight. The 1230 nm light exits OPO 2108 and is converted by SHG unit2138 into 615 nm red light. The 532.5 nm green light that is not lost inOPO 2108 or converted into blue or red light by OPO 2108, exits OPO2108.

Nd:YLF laser 2148 generates light at 1047 nm which enters Nd:YLF gainmodule 2150, is optically amplified by gain module 2150, then enters SHGunit 2152 where it is converted to green light at 523.5 nm and entersOPO 2110. A fraction of the 523.5 nm light in OPO 2110 is converted to880 nm and a fraction is converted to 1288 nm. The 880 nm light exitsOPO 2110 and is converted by SHG unit 2144 into 440 nm blue light. The1288 nm light exits OPO 2110 and is converted by SHG unit 2146 into 644nm red light. The 532.5 nm green light that exits OPO 2110 goes intobeam dump 2112.

Nd:YAP laser 2128 generates light at 1079.5 nm which enters Nd:YAP gainmodule 2130, is optically amplified by gain module 2130, then enters SHGunit 2142 where it is converted to green light at 539.7 nm.

The 455 nm blue light from OPO 2108 and the 440 nm blue light from OPO2110 pass through blue filter 2114 and form light beam 2120. The 523.5nm green light from OPO 2108 and the 539.7 nm green light from SHG unit2142 pass through green filter 2116 and form light beam 2122. The 615 nmred light from OPO 2108 and the 644 nm red light from OPO 2110 passthrough red filter 2118 and form light beam 2124. Light beams 2120,2122, and 2124 are combined together to form light beam 2100. The blue,green, and red filters may be band pass filters that block visiblecolors and infrared light of wavelengths outside the pass band.

Nd:YLF laser 2100, Nd:YLF gain module 2102, SHG unit 2104, OPO 2108, SHGunit 2136, and SHG unit 2138 form light generation unit 2132 which isused to form images for the left eye. Nd:YLF laser 2148, Nd:YLF gainmodule 2150, SHG unit 2152, OPO 2110, SHG unit 2144, beam dump 2112, SHGunit 2146, Nd:YAP laser 2128, ND:YAP gain module 2130, and SHG unit2142, form light generation unit 2134 which is used to form images forthe right eye. Alternately, light generation unit 2132 may be used forthe right eye and light generation unit 2134 may be used for the lefteye.

Light system 1918 as described in FIG. 21 may produce at least 50 wattsof optical power including all six colors of output in light beams 2120,2122, and 2124. If the total optical power output is 100 watts, Nd:YLFlasers 2100 and 2148 each produce about 2 watts of 1047 nm light andNd:YLF gain module 2102 and 2150 each produce about 300 watts of 1047 nmlight. Nd:YAP laser 2128 produces about 2 watts of 1079.5 nm light andNd:YAP gain module 2130 produce about 60 watts of 1079.5 nm light.

FIG. 22 shows a method of generating light based on two lasers whichcorresponds to the laser light system shown in FIG. 20. In step 2200, afirst beam of infrared laser light is generated. In step 2202, the firstbeam of infrared light is amplified. In step 2204, a first green lightis generated by converting the first beam of infrared light to greenlight. In step 2206, the first green light is switched between left eyeand right eye beams. In step 2208, the left, first green light isconverted to second and third beams of infrared light. In step 2210,first red light and first blue light are generated by converting thesecond and third beams of infrared light. In step 2212, the remainingfirst green light, first red light, and first blue light are output bythe method. In step 2214, the right, first green light is converted tofourth and fifth beams of infrared light. In step 2216, second red lightand second blue light are generated by converting the fourth and fifthbeams of infrared light. In step 2218, the remaining right, first greenlight is absorbed by a beam dump. In step 2222, a sixth beam of infraredlaser light is generated. In step 2224, the sixth beam of infrared lightis amplified. In step 2226, a second green light is generated byconverting the sixth beam of infrared light to green light. In step2220, the second green light, second red light, and second blue lightare output by the method.

FIG. 23 shows a method of generating light based on three lasers whichcorresponds to the laser light system shown in FIG. 21. In step 2300, afirst beam of infrared laser light is generated. In step 2302, the firstbeam of infrared light is amplified. In step 2304, a first green lightis generated by converting the first beam of infrared light to greenlight. In step 2306, the first green light is converted to second andthird beams of infrared light. In step 2308, first red light and firstblue light are generated by converting the second and third beams ofinfrared light. In step 2310, the remaining first green light, first redlight, and first blue light are output by the method. In step 2312, afourth beam of infrared laser light is generated. In step 2314, thefourth beam of infrared light is amplified. In step 2316, a second greenlight is generated by converting the first beam of infrared light togreen light. In step 2318, the second green light is converted to fifthand sixth beams of infrared light. In step 2320, second red light andsecond blue light are generated by converting the fifth and sixth beamsof infrared light. In step 2322, the remaining second green light isabsorbed by a beam dump. In step 2326, a seventh beam of infrared laserlight is generated. In step 2328, the seventh beam of infrared light isamplified. In step 2330, a third green light is generated by convertingthe seventh beam of infrared light to green light. In step 2324, thethird green light, second red light, and second blue light are output bythe method.

OPOs such as those shown in elements 2008, 2010, 2108, and 2110 in FIGS.20 and 21 use parametric amplification to convert a fraction of theinput light into two other wavelengths. By suitably designing the OPOand controlling operation parameters such as temperature, when the inputlight is green, the outputs may be blue and red, thus making all threecolors required for a full-color projection display. By mixing red,green, and blue light, other colors may be generated in projector 1800.In FIG. 20, OPO 2008 and OPO 2010 are controlled at differenttemperatures in order to make different output wavelengths. In FIG. 21,OPO 2108 and OPO 2110 are controlled at different temperatures in orderto make different output wavelengths. In order to achieve the desiredcolor of white light (also called the white point), there must be acertain fraction of red light, blue light, and green light mixedtogether. If the OPO under converts green light into blue light and redlight, some green light must be wasted when forming white light. If theOPO over converts green light into blue light and red light, the OPOconversion efficiency can be controlled by slightly detuning the OPO inorder to attain the desired white point. An optical sensor such as acolor sensor may be employed for real time sensing of the white pointand real time control of the OPO tuning to maintain the desired whitepoint over time. An intensity sensor may likewise be employed for realtime sensing of the output power and real time control of the lightsystem output to maintain the desired output intensity over time.Alternately, each color channel can be monitored separately with anoptical sensor to achieve the desired white point and overall outputintensity. The outputs from the color or intensity sensors may beelectrical signals that feed into electronic circuits that control theintensity of each color channel.

The OPOs shown in FIGS. 20 and 21 may be constructed from LBO crystalsoperated in the type I, noncritical phase matching mode. In thiscondition, the temperature of OPO 2008 and OPO 2108 may be 131 degreesCelsius and the temperature of OPO 2010 and OPO 2110 may be 137 degreesCelsius. The resultant output wavelengths of the laser light systems inFIGS. 20 and 21 may be 523.5 nm, 539.7 nm, 615 nm, 644 nm, 455 nm, and440 nm. The wavelengths may vary 1 or 2 nm from these values withoutchanging the essential nature of operation.

Laser light systems enable light to be generated in the narrow bandsrequired by the spectrally selective stereoscopic glasses. This allowshigh brightness images even when projected on large screen sizes.Conventional projector light sources, such as Xenon bulbs, must befiltered to produce narrow bands and thus are highly inefficient leadingto brightness limitations, especially for large screen sizes. Laserlight systems also produce narrower bands than filtered Xenon sourceswhich allow the colors to be better placed for achieving optimal colorgamut and optimal isolation between the two eyes to reduce ghosting.

Gain modules such as those shown in FIGS. 20 and 21 use stimulatedemission from a gain medium to amplify light so that the output beam hashigher power than the input beam. The detailed operation of gain modulesis described in U.S. Pat. No. 5,774,489, the complete disclosure ofwhich is incorporated herein by reference. Gain modules include one ormore stages of gain where each stage of gain includes one gain slab andassociated pump lasers.

SHG units such as those shown in SHG units 2004, 2036, 2038, 2040, 2042,2044, 2104, 2136, 2138, 2152, 2142, 2144, and 2146 in FIGS. 20 and 21use nonlinear optical processes to convert the wavelength of theoriginal light into a harmonic wavelength such as half the originalwavelength. The detailed operation of SHG units is described in U.S.Pat. No. 4,019,159, the complete disclosure of which is incorporatedherein by reference.

The wavelengths of light used for spectrally-selective stereoscopicimaging can be any wavelengths that can be filtered such that one set ofwavelengths arrives only at one eye, and the other set arrives only atthe other eye. There may be one wavelength for each eye, two wavelengthsfor each eye, three wavelengths for each eye, or more. The case of threewavelengths for each eye (six colors total) is the minimum number ofwavelengths to produce a full color image. In the case of fourwavelengths for each eye, a larger gamut of colors can be expressed thanin the case of three wavelengths for each eye. The wavelengths describedin FIGS. 20 and 21 may be adjusted to fit the requirements of theindividual projection systems or available wavelength selective glasses.

The FWHM bandwidths of the wavelength bands may be as narrow as 0.05 nmor as wide as 20 nm depending on the types of lasers used and theconstruction of the OPOs, gain modules, SHG units, and filters. Narrowwavelength bands are generally subject to increased visible speckle whencompared to wider bands. Laser light systems allow a larger color gamutthan conventional light sources such as Xenon lamps. Particularly in thegreen region of the color gamut, an alternate color space is availablebecause laser light systems allow substantial generation of light at532.5 nm rather than the typical green wavelength of Xenon lamps whichis centered near 546 nm.

The color gamut of the laser light system may be tuned by changing theoperation temperature of the nonlinear element in the OPO or otherparameter of the OPO such that the red and blue wavelengths are shifted.Typically, when the red wavelength shifts towards the center of thevisible wavelength band, the blue also shifts towards the center of thevisible. This makes a direct tradeoff between size of the color gamutand brightness because when the red and blue are shifted towards thecenter of the visible wavelength band, the human eye perceives higherbrightness according to the photopic sensitivity curve whereas when thered and blue are shifted away from the center of the visible wavelengthband, the color gamut is larger, but the human eye perceives lessbrightness.

Q-switched lasers are often best to achieve the high power densitiesrequired for the nonlinear effects in OPOs and SHG units. TheQ-switching frequency may be in the range of 20 kHz to 300 kHz, withpulse widths of 5 ns to 100 ns. Continuous wave (also called oscillator)lasers may alternately be used in some cases such as is shown in lasers2028 and 2128 of FIGS. 20 and 21.

Spectrally selective glasses, as shown in FIG. 18, allow left eye imagesto be transmitted to only the left eye and right eye images to betransmitted to only the right eye. The narrow bands of light from laserlight systems enable the efficient use of narrow transmission bands inthe spectrally selective glasses which has the benefits of reducedghosting and improved rejection of ambient light. Glasses may be active(shutter) glasses or passive (non-shutter) glasses. In the case ofactive glasses, the glasses may be synchronized with the pulses of thelaser light system to attain high system efficiency.

Various projector types can be used with laser light systems. Inaddition to the DMD design shown in FIG. 19, laser light systems can beused with LCOS light valves, LCD light valves, and other types of lightvalves. The design of the projector will be different for each type oflight valve, but the laser light system may generally be substituted fora xenon or other broad band light source with very few or no changes inthe projector design. This allows after-market substitution of the laserlight system. If the projector is to be designed specifically foroperation with a laser light system, the projector may be designed witha low etendue that matches the low etendue of the laser light system.This may allow the projector to generate higher contrast images comparedto a projector with higher etendue. Polarized laser light systems mayallow more efficient light throughput when combined with light valvesthat operate with polarized light such as LCD and LCOS light valves. Thepolarization may be linear or circular.

In another aspect of the optical system shown in FIG. 1, dualillumination is used to illuminate two parts of each SLM in a projector.First laser light source 102 may be used to illuminate the first part ofthe SLMs and second laser light source 116 may be used to illuminate thesecond part of the SLMs. Dual illumination may allow reduced complexityin the optical system compared to using separate sets of SLMs. Specificembodiments are described in the following paragraphs but are not meantto be limiting in any way.

Split image projection has the advantage of using fewer SLMs and otheroptical components compared to simultaneous projection. Split imageprojection also has the advantage of not requiring active glasses suchas those used in time sequential projection. Technological progressleads towards ever higher and higher pixel counts per SLM which alsotends to favor using more than one image per SLM while still allowingsufficient pixels in each image to achieve high resolutions such as1920×1080 pixels (full high definition) which is also known as 2K. Veryhigh resolution 4K SLMs, (which may be 4096×2160 resolution) areavailable for cinema applications. Two 2K images may be processed on twoparts of one 4K SLM. In the case of stereoscopic projection, one of the2K images may be viewed by the left eye, and the other 2K image may beviewed by the right eye. Dual illumination allows one low-etendue lightsource to illuminate one part of the SLM, and a second low-etendue lightsource to illuminate a second part of the same SLM. Other advantages oflow-etendue light sources will also be seen in the following examples.

FIG. 24 shows a projector optical design with dual illumination usingLCOS SLMs. First light source 2400 produces first beam segment 2402which is spread by first lens system 2404 to make second beam segment2406. Second beam segment 2406 is homogenized by first mixing rod 2408to produce third beam segment 2410. Third beam segment 2410 iscollimated by second lens system 2412 to form fourth beam segment 2414.Fourth beam segment 2414 partially reflects from first DBS 2432 to formfifth beam segment 2472 and partially transmits to form sixth beamsegment 2436. Fifth beam segment 2472 reflects from first mirror 2475 toform seventh beam segment 2476. Seventh beam segment 2476 enters firstPBS 2480 and is reflected to form eighth beam segment 2484. Eighth beamsegment 2484 is processed by first SLM 2485 which rotates thepolarization of each pixel depending on the desired brightness of thepixel and reflects eighth beam segment 2484 back along its input path toreenter first PBS 2480. On a pixel-by-pixel basis, if the polarizationis not changed relative to the input beam, the light reflects insidefirst PBS 2480 to go back towards first light source 2400. If thepolarization has been changed relative to the input beam, some or all ofthe light (depending on how much the polarization has been changed)passes through first PBS 2480 to form ninth beam segment 2486.

Sixth beam segment 2436 partially reflects from second DBS 2438 to formtenth beam segment 2458 and partially transmits to form eleventh beamsegment 2442. Tenth beam segment 2458 enters second PBS 2460 and isreflected to form twelfth beam segment 2464. Twelfth beam segment 2464is processed by second SLM 2466 which rotates the polarization of eachpixel depending on the desired brightness of the pixel and reflectstwelfth beam segment 2464 back along its input path to reenter secondPBS 2460. On a pixel-by-pixel basis, if the polarization is not changedrelative to the input beam, the light reflects inside second PBS 2460 togo back towards first light source 2400. If the polarization has beenchanged relative to the input beam, some or all of the light (dependingon how much the polarization has been changed) passes through second PBS2460 to form thirteenth beam segment 2468.

Eleventh beam segment 2442 enters third PBS 2444 and is reflected toform fourteenth beam segment 2446. Fourteenth beam segment 2446 isprocessed by third SLM 2450 which rotates the polarization of each pixeldepending on the desired brightness of the pixel and reflects fourteenthbeam segment 2446 back along its input path to reenter third PBS 2444.On a pixel-by-pixel basis, if the polarization is not changed relativeto the input beam, the light reflects inside third PBS 2444 to go backtowards first light source 2400. If the polarization has been changedrelative to the input beam, some or all of the light (depending on howmuch the polarization has been changed) passes through third PBS 2444 toform fifteenth beam segment 2454.

First beam combiner 2488 combines ninth beam segment 2486, thirteenthbeam segment 2468, and fifteenth beam segment 2454 to form sixteenthbeam segment 2490. Sixteenth beam segment 2490 reflects from secondminor 2491 to form seventeenth beam segment 2493. Seventeenth beamsegment 2493 reflects from third DBS 2492 to form beam segment 2494.

Second light source 2416 produces eighteenth beam segment 2418 which isspread by third lens system 2420 to make nineteenth beam segment 2422.Nineteenth beam segment 2422 is homogenized by second mixing rod 2424 toproduce twentieth beam segment 2426. Twentieth beam segment 2426 iscollimated by fourth lens system 2428 to form twenty-first beam segment2430. Twenty-first beam segment 2430 partially reflects from first DBS2432 to form twenty-second beam segment 2474 and partially transmits toform twenty-third beam segment 2434. Twenty-second beam segment 2474reflects from first minor 2475 to form twenty-fourth beam segment 2478.Twenty-fourth beam segment 2478 enters first PBS 2480 and is reflectedto form twenty-fifth beam segment 2482. Twenty-fifth beam segment 2482is processed by first SLM 2485, which rotates the polarization of eachpixel depending on the desired brightness of the pixel and reflectstwenty-fifth beam segment 2482 back along its input path to reenterfirst PBS 2480. On a pixel-by-pixel basis, if the polarization is notchanged relative to the input beam, the light reflects inside first PBS2480 to go back towards first light source 2400. If the polarization hasbeen changed relative to the input beam, some or all of the light(depending on how much the polarization has been changed) passes throughfirst PBS 2480 to form twenty-sixth beam segment 2487.

Twenty-third beam segment 2434 partially reflects from second DBS 2438to form twenty-seventh beam segment 2456 and partially transmits to formtwenty-eighth beam segment 2440. Twenty-seventh segment 2456 enterssecond PBS 2460 and is reflected to form twenty-ninth beam segment 2462.Twenty-ninth beam segment 2462 is processed by second SLM 2466, whichrotates the polarization of each pixel depending on the desiredbrightness of the pixel and reflects twenty-ninth beam segment 2462 backalong its input path to reenter second PBS 2460. On a pixel-by-pixelbasis, if the polarization is not changed relative to the input beam,the light reflects inside second PBS 2460 to go back towards first lightsource 2400. If the polarization has been changed relative to the inputbeam, some or all of the light (depending on how much the polarizationhas been changed) passes through second PBS 2460 to form thirtieth beamsegment 2470.

Twenty-eighth beam segment 2440 enters third PBS 2444 and is reflectedto form thirty-first beam segment 2448. Thirty-first beam segment 2448is processed by third SLM 2450, which rotates the polarization of eachpixel depending on the desired brightness of the pixel and reflectsthirty-first beam segment 2448 back along its input path to reenterthird PBS 2444. On a pixel-by-pixel basis, if the polarization is notchanged relative to the input beam, the light reflects inside third PBS2444 to go back towards first light source 2400. If the polarization hasbeen changed relative to the input beam, some or all of the light(depending on how much the polarization has been changed) passes throughthird PBS 2444 to form thirty-second beam segment 2452.

First beam combiner 2488 combines twenty-sixth beam segment 2487,thirtieth beam segment 2470, and thirty-second beam segment 2452 to formthirty-third beam segment 2489. Thirty-third beam segment 2489 passesthrough third DBS 2492 to combine with seventeenth beam segment 2493 informing thirty-fourth beam segment 2494. Thirty-fourth beam segment 2494passes through fifth lens system 2495 to form thirty-fifth beam segment2496 which passes outside of the projector to make a viewable image on aprojection screen (not shown).

First beam combiner 2488 may be an X-prism. Second mirror 2491 and thirdDBS 2492 form second beam combiner 2497. First lens system 2404, secondlens system 2412, third lens system 2420, fourth lens system 2428, andfifth lens system 2495 may be formed from a single lens or any number oflenses that guide the light beams into the desired positions. The sizesof components and distances between components are not shown to scale inFIG. 24. Some optical components may be positioned against other opticalcomponents so that there is no gap between the components. Auxiliaryoptical components such as polarizers, relay lenses, skew ray plates,polarization rotation plates, and trim filters are not shown in FIG. 24.The three SLMs shown in FIG. 24 may be each assigned to a primary colorso that one is red, one is green, and one is blue. First light source2400 may output sub-bands red 1, green 1, and blue 1 whereas secondlight source 2416 may output sub-bands red 2, green 2, and blue 2. FirstDBS 2432 may reflect blue while passing green and red. Second DBS 2438may reflect green while passing red. Third DBS 2492 may reflectsub-bands red 1, green 1, and blue 1 while passing sub-bands red 2,green 2, and blue 2. First light source 2400 and second light source2416 may output polarized light.

FIG. 25 shows a projector optical design with dual illumination usingDMD SLMs. First light source 2502 produces first beam segment 2504 whichis spread by first lens system 2506 to make second beam segment 2508.Second beam segment 2508 is homogenized by first mixing rod 2510 toproduce third beam segment 2512. Third beam segment 2512 is collimatedby second lens system 2514 to form fourth beam segment 2516. Fourth beamsegment 2516 enters first subprism 2534 and reflects from the interfaceof first subprism 2534 and second subprism 2568 to form fifth beamsegment 2538. Fifth beam segment 2538 partially reflects from theinterface between third subprism 2540 and fourth subprism 2556 and thenfrom the entrance face of third subprism 2540 to form sixth beam segment2544. Fifth beam segment 2538 also partially transmits from theinterface between third subprism 2540 and fourth subprism 2556 andpartially transmits from the interface between fourth subprism 2556 andfifth subprism 2548 to form seventh beam segment 2552. Fifth beamsegment 2538 also partially reflects from the interface between fourthsubprism 2556 and fifth subprism 2548 and then reflects from theinterface between fourth subprism 2556 and third subprism 2540 to formeighth beam segment 2560. Sixth beam segment 2544 is processed by firstSLM 2546 which flips micromirrors for each pixel depending on thedesired brightness of the pixel. For darker pixels, more light isdirected back through the prism systems until the light is absorbed in abeam dump (not shown) and for brighter pixels, more light is directed tothe output path which reflects from the entrance face of third subprism2540 and then from the interface between third subprism 2540 and fourthsubprism 2556 to form ninth beam segment 2566.

Seventh beam segment 2552 is processed by second SLM 2554 which flipsmicromirrors for each pixel depending on the desired brightness of thepixel. For darker pixels, more light is directed back through the prismsystems until the light is absorbed in a beam dump (not shown) and forbrighter pixels, more light is directed to the output path which passesthrough the interface between fifth subprism 2548 and fourth subprism2556, then passes through the interface between fourth subprism 2556 andthird subprism 2540 to join sixth beam segment 2544 in forming ninthbeam segment 2566.

Eighth beam segment 2560 is processed by third SLM 2562 which flipsmicromirrors for each pixel depending on the desired brightness of thepixel. For darker pixels, more light is directed back through the prismsystems until the light is absorbed in a beam dump (not shown) and forbrighter pixels, more light is directed to the output path whichreflects from the interface between fourth subprism 2556 and thirdsubprism 2540, then reflects from the interface between fourth subprism2556 and fifth subprism 2548, then passes through the interface betweenfourth subprism 2556 and third subprism 2540 to join sixth beam segment2544 and seventh beam segment 2552 in forming ninth beam segment 2566.

Ninth beam segment 2566 passes through first subprism 2534 and secondsubprism 2568 to form tenth beam segment 2572. Tenth beam segment 2572passes through first DBS 2578 to form eleventh beam segment 2580.

Second light source 2518 produces twelfth beam segment 2520 which isspread by third lens system 2522 to make thirteenth beam segment 2524.Thirteenth beam segment 2524 is homogenized by second mixing rod 2526 toproduce fourteenth beam segment 2528. Fourteenth beam segment 2528 iscollimated by fourth lens system 2530 to form fifteenth beam segment2532. Fifteenth beam segment 2532 enters first subprism 2534 andreflects from the interface of first subprism 2534 and second subprism2568 to form sixteenth beam segment 2536. Sixteenth beam segment 2536partially reflects from the interface between third subprism 2540 andfourth subprism 2556 and then from the entrance face of third subprism2540 to form seventeenth beam segment 2542. Sixteenth beam segment 2536also partially transmits from the interface between third subprism 2540and fourth subprism 2556 and partially transmits from the interfacebetween fourth subprism 2556 and fifth subprism 2548 to form eighteenthbeam segment 2550. Sixteenth beam segment 2536 also partially reflectsfrom the interface between fourth subprism 2556 and fifth subprism 2548and then reflects from the interface between fourth subprism 2556 andthird subprism 2540 to form nineteenth beam segment 2558. Seventeenthbeam segment 2542 is processed by first SLM 2546 which flipsmicromirrors for each pixel depending on the desired brightness of thepixel. For darker pixels, more light is directed back through the prismsystems until the light is absorbed in a beam dump (not shown) and forbrighter pixels, more light is directed to the output path whichreflects from the entrance face of third subprism 2540 and then from theinterface between third subprism 2540 and fourth subprism 2556 to formtwentieth beam segment 2564.

Eighteenth beam segment 2550 is processed by second SLM 2554 which flipsmicromirrors for each pixel depending on the desired brightness of thepixel. For darker pixels, more light is directed back through the prismsystems until the light is absorbed in a beam dump (not shown) and forbrighter pixels, more light is directed to the output path which passesthrough the interface between fifth subprism 2548 and fourth subprism2556, then passes through the interface between fourth subprism 2556 andthird subprism 2540 to join seventeenth beam segment 2542 in formingtwentieth beam segment 2564.

Nineteenth beam segment 2558 is processed by third SLM 2562 which flipsmicromirrors for each pixel depending on the desired brightness of thepixel. For darker pixels, more light is directed back through the prismsystems until the light is absorbed in a beam dump (not shown) and forbrighter pixels, more light is directed to the output path whichreflects from the interface between fourth subprism 2556 and thirdsubprism 2540, then reflects from the interface between fourth subprism2556 and fifth subprism 2548, then passes through the interface betweenfourth subprism 2556 and third subprism 2540 to join seventeenth beamsegment 2542 and eighteenth beam segment 2550 in forming twentieth beamsegment 2564.

Twentieth beam segment 2564 passes through first subprism 2534 andsecond subprism 2568 to form twenty-first beam segment 2570.Twenty-first beam segment 2570 reflects from first mirror 2574 to formtwenty-second beam segment 2576 and then reflects from first DBS 2578 tojoin tenth beam segment 2572 in forming eleventh beam segment 2580.Eleventh beam segment 2580 passes through fifth lens system 2582 to formtwenty-third beam segment 2584 which passes outside of the projector tomake a viewable image on a projection screen (not shown).

First subprism 2534 and second subprism 2568 form TIR prism 2588. Thirdsubprism 2540, fourth subprism 2556, and fifth subprism 2548 formPhilips prism 2586. Minor 2574 and DBS 2578 form beam combiner 2590.First lens system 2506, second lens system 2514, third lens system 2522,fourth lens system 2530, and fifth lens system 2582 may be formed from asingle lens or any number of lenses that guide the light beams into thedesired positions. The sizes of components and distances betweencomponents are not shown to scale in FIG. 25. Some optical componentsmay be positioned against other optical components so that there is nogap between the components. Auxiliary optical components such aspolarizers, relay lenses, skew ray plates, polarization rotation plates,and trim filters are not shown in FIG. 25. The three SLMs shown in FIG.25 may be each assigned to a primary color so that one is red, one isgreen, and one is blue. First light source 2502 may output sub-bands red1, green 1, and blue 1 whereas second light source 2518 may outputsub-bands red 2, green 2, and blue 2. The interface between thirdsubprism 2540 and fourth subprism 2556 may reflect blue while passinggreen and red. The interface between fourth subprism 2556 and fifthsubprism 2548 may transmit green while reflecting red. First DBS 2578may transmit sub-bands red 1, green 1, and blue 1 while reflectingsub-bands red 2, green 2, and blue 2.

FIG. 26 shows a projector optical design with dual illumination usingtransmissive LCD SLMs. First light source 2600 produces first beamsegment 2602 which is spread by first lens system 2604 to make secondbeam segment 2606. Second beam segment 2606 is homogenized by firstmixing rod 2608 to produce third beam segment 2610. Third beam segment2610 is collimated by second lens system 2612 to form fourth beamsegment 2614. Fourth beam segment 2614 partially reflects from first DBS2632 to form fifth beam segment 2664 and partially transmits to formsixth beam segment 2636. Fifth beam segment 2664 reflects from firstminor 2668 to form seventh beam segment 2672. Seventh beam segment 2672is processed by first SLM 2674 which rotates the polarization of eachpixel depending on the desired brightness of the pixel. On apixel-by-pixel basis, depending on the amount of polarization rotation,the light is transmitted or absorbed to a varying degree by a polarizer(not shown) to form eighth beam segment 2676.

Sixth beam segment 2636 partially reflects from second DBS 2638 to formninth beam segment 2681 and partially transmits to form tenth beamsegment 2640. Ninth beam segment 2681 is processed by second SLM 2682which rotates the polarization of each pixel depending on the desiredbrightness of the pixel. On a pixel-by-pixel basis, depending on theamount of polarization rotation, the light is transmitted or absorbed toa varying degree by a polarizer (not shown) to form eleventh beamsegment 2684.

Tenth beam segment 2640 reflects from second minor 2644 to form twelfthbeam segment 2648. Twelfth beam segment 2648 reflects from third mirror2650 to form thirteenth beam segment 2654. Thirteenth beam segment 2654is processed by third SLM 2656 which rotates the polarization of eachpixel depending on the desired brightness of the pixel. On apixel-by-pixel basis, depending on the amount of polarization rotation,the light is transmitted or absorbed to a varying degree by a polarizer(not shown) to form fourteenth beam segment 2660.

First beam combiner 2662 combines eighth beam segment 2676, eleventhbeam segment 2684, and fourteenth beam segment 2660 to form fifteenthbeam segment 2685. Fifteenth beam segment 2685 reflects from fourthmirror 2687 to form sixteenth beam segment 2688. Sixteenth beam segment2688 reflects from third DBS 2690 to form seventeenth beam segment 2691.

Second light source 2616 produces eighteenth beam segment 2618 which isspread by third lens system 2620 to make nineteenth beam segment 2622.Nineteenth beam segment 2622 is homogenized by second mixing rod 2624 toproduce twentieth beam segment 2626. Twentieth beam segment 2626 iscollimated by fourth lens system 2628 to form twenty-first beam segment2630. Twenty-first beam segment 2630 partially reflects from first DBS2632 to form twenty-second beam segment 2666 and partially transmits toform twenty-third beam segment 2634. Twenty-second beam segment 2666reflects from first minor 2668 to form twenty-fourth beam segment 2670.Twenty-fourth beam segment 2670 is processed by first SLM 2674 whichrotates the polarization of each pixel depending on the desiredbrightness of the pixel. On a pixel-by-pixel basis, depending on theamount of polarization rotation, the light is transmitted or absorbed toa varying degree by a polarizer (not shown) to form twenty-fifth beamsegment 2678.

Twenty-third beam segment 2634 partially reflects from second DBS 2638to form twenty-sixth beam segment 2680 and partially transmits to formtwenty-seventh beam segment 2642. Twenty-sixth beam segment 2680 isprocessed by second SLM 2682 which rotates the polarization of eachpixel depending on the desired brightness of the pixel. On apixel-by-pixel basis, depending on the amount of polarization rotation,the light is transmitted or absorbed to a varying degree by a polarizer(not shown) to form twenty-eighth beam segment 2683.

Twenty-seventh beam segment 2642 reflects from second mirror 2644 toform twenty-ninth beam segment 2646. Twenty-ninth beam segment 2646reflects from third minor 2650 to form thirtieth beam segment 2652.Thirtieth beam segment 2652 is processed by third SLM 2656 which rotatesthe polarization of each pixel depending on the desired brightness ofthe pixel. On a pixel-by-pixel basis, depending on the amount ofpolarization rotation, the light is transmitted or absorbed to a varyingdegree by a polarizer (not shown) to form thirty-first beam segment2658.

First beam combiner 2662 combines twenty-fifth beam segment 2678,twenty-eighth beam segment 2683, and thirty-first beam segment 2658 toform thirty-second beam segment 2686. Thirty-second beam segment 2686passes through third DBS 2690 to combine with sixteenth beam segment2688 in forming seventeenth beam segment 2691. Seventeenth beam segment2691 passes through fifth lens system 2692 to form thirty-third beamsegment 2693 which passes outside of the projector to make a viewableimage on a projection screen (not shown).

First beam combiner 2662 may be an X-prism. Second mirror 2687 and thirdDBS 2690 form second beam combiner 2694. First lens system 2604, secondlens system 2612, third lens system 2620, fourth lens system 2628, andfifth lens system 2692 may be formed from a single lens or any number oflenses that guide the light beams into the desired positions. The sizesof components and distances between components are not shown to scale inFIG. 26. Some optical components may be positioned against other opticalcomponents so that there is no gap between the components. Auxiliaryoptical components such as polarizers, relay lenses, skew ray plates,polarization rotation plates, and trim filters are not shown in FIG. 26.The three SLMs shown in FIG. 26 may be each assigned to a primary colorso that one is red, one is green, and one is blue. First light source2600 may output sub-bands red 1, green 1, and blue 1 whereas secondlight source 2616 may output sub-bands red 2, green 2, and blue 2. FirstDBS 2632 may reflect blue while passing green and red. Second DBS 2638may reflect green while passing red. Third DBS 2690 may reflectsub-bands red 1, green 1, and blue 1 while passing sub-bands red 2,green 2, and blue 2. First light source 2600 and second light source2616 may output polarized light.

FIG. 27 shows a portrait-oriented SLM with two images located one abovethe other. First image 2702 is formed in one part of SLM 2700 and secondimage 2704 is formed in another, distinct part of SLM 2700. First image2702 and second image 2704 are located such that most of the un-usedpixels are above and below each image. In the case of stereoscopicsystems, first image 2702 may be the left eye image and second image2704 may be the right eye image.

FIG. 28 shows a landscape-oriented SLM with two images located one abovethe other. First image 2802 is formed in one part of SLM 2800 and secondimage 2804 is formed in another, distinct part of SLM 2800. First image2802 and second image 2804 are located such that most of the un-usedpixels are on the left and right of each image. In the case ofstereoscopic systems, first image 2802 may be the left eye image andsecond image 2804 may be the right eye image.

FIG. 29 shows a portrait-oriented SLM with two images far apart andlocated one above the other. First image 2902 is formed in one part ofSLM 2900 and second image 2904 is formed in another, distinct part ofSLM 2900. First image 2902 and second image 2904 are located such thatmost of the un-used pixels are between the two images. In the case ofstereoscopic systems, first image 2902 may be the left eye image andsecond image 2904 may be the right eye image. By placing first image2902 far from second image 2904 an increased guard band is formedbetween the two images that may reduce the amount of cross-talk or lightspillage between the two images.

FIG. 30 shows a landscape-oriented SLM with two images located on theleft and right of each other so that they form a central band across thehorizontal center of the SLM. First image 3002 is formed in one part ofSLM 3000 and second image 3004 is formed in another, distinct part ofSLM 3000. First image 3002 and second image 3004 are located such thatmost of the un-used pixels are formed into one band above the images andone band below the images. In the case of stereoscopic systems, firstimage 3002 may be the left eye image and second image 3004 may be theright eye image.

FIG. 31 shows a landscape-oriented SLM with two images located onediagonal to the other. First image 3102 is formed in one part of SLM3100 and second image 3104 is formed in another, distinct part of SLM3100. First image 3102 and second image 3104 are located such that mostof the un-used pixels are above and below the two images in diagonallyopposite corners. In the case of stereoscopic systems, first image 3102may be the left eye image and second image 3104 may be the right eyeimage.

FIG. 32 shows a landscape-oriented SLM with an anamorphic pattern ofpixels with one image located above the other. First image 3202 isformed in one part of SLM 3200 and second image 3204 is formed inanother, distinct part of SLM 3200. First image 3202 and second image3204 use substantially all of the pixels in SLM 3200. An anamorphic lensmay be used to compress the horizontal axis (relative to the verticalaxis) or expand the vertical axis (relative to the horizontal axis) suchthat the final viewable images are formed with the desired aspect ratio.In the case of stereoscopic systems, first image 3202 may be the lefteye image and second image 3204 may be the right eye image.

FIG. 33 shows a landscape-oriented SLM with a checkerboard pattern ofpixels. Pixels of the first image on SLM 3300 are shown cross-hatchedsuch as pixel 3302 and pixels of the second image on SLM 3300 are shownnot cross-hatched such as pixel 3304. A 31×15 array of pixels is shownfor clarity, but SLMs typically have many more pixels to form highresolution images. The checkerboard pattern uses substantially all thepixels of the SLM. In the case of stereoscopic systems, the first imagemay be the left eye image and second image may be the right eye image.

FIG. 34 shows low etendue illumination of an SLM and an opticalcomponent compared to high etendue illumination of the same SLM andoptical component. First beam segment 3400 passes through opticalcomponent 3402 to form second beam segment 3404. Second beam segment3404 passes through SLM 3406 to form third beam segment 3408.Alternatively, fourth beam segment 3410 passes through optical component3402 to form fifth beam segment 3412. Fifth beam segment 3412 passesthrough SLM 3406 to form sixth beam segment 3414. First beam segment3400, second beam segment 3404, and third beam segment 3408 have highetendue. Fourth beam segment 3410, fifth beam segment 3412, and sixthbeam segment 3414 have low etendue. First beam segment 3400, second beamsegment 3404, and third beam segment 3408 can be seen to have higherangles of incidence for rays near the edges of the beam segments. Fourthbeam segment 3410, fifth beam segment 3412, and sixth beam segment 3414can be seen to have lower angles of incidence for rays near the edges ofthe beam segments. Optical component 3402 may be any component thatprocesses light such as a polarizer, skew ray plate, polarizationrotation plate, interference filter, beamsplitter, minor, or lensassembly. Skew ray plates are used to compensate the polarization stateof rays at high angle of incidence. Polarization rotation plates make acontrolled change in polarization such as changing linear polarizationto circular polarization. SLM 3406 may be any sort of SLM such as DMD,LCD, or LCOS. Optical component 3402 and SLM 3406 are shown operating intransmission, but may alternatively operate in reflection. Opticalcomponent 3402 is shown to in the light path before SLM 3406, butalternatively, optical component 3402 may be after SLM 3406. Theincluded angles of beam segments shown in FIG. 34 are for illustrativepurposes only. The actual beam angles may be larger or smaller dependingon the design of the actual optical system.

FIG. 35 shows a flow chart of a method of dual illumination. In thismethod, an SLM is illuminated by two light sources. In step 3500, afirst beam of light is generated. In step 3502, the first beam of lightis processed by the first part of an SLM. In step 3504, a second beam oflight generated. In step 3506, the second beam of light is processed bythe second part of the same SLM. In optional step 3508, the first beamof light after processing is combined with the second beam of lightafter processing.

When considering a light source, etendue is an optical property thatcharacterizes how spread out the light beam is in both area and angle.In simple terms, the approximate etendue of a light source may becomputed by multiplying the emitting area of the source by the solidangle that the light beam subtends. Lasers have low etendue whereas arclamps, filament lamps, and LEDs have high etendue. If the light sourcehas sufficiently low etendue, it is possible to focus light through asubsequent optical system with high efficiency. Laser light sourcesenable the independent illumination of more than one part of an SLM withhigh brightness. As an example, the beam from a semiconductor laser mayhave a cross-sectional area of 1 mm² and a beam divergence of 10milliradians which makes an etendue of approximately 0.01 mm² sr. Mostlasers have etendues less than 0.1 mm² sr, which allows effectiveillumination of multiple parts of an SLM. An example of a high etenduelight source is an arc lamp which may have an emitting area of 3 mm² anda beam divergence of 12.6 radians which makes an etendue ofapproximately 38 mm² sr.

When considering an optical system which accepts light from a lightsource, etendue is the optical property that characterizes how muchlight the optical system can accept in both aperture area and angle. Insimple terms, the approximate etendue of an optical system may becomputed by multiplying the area of the entrance pupil by the solidangle of the light path as seen from the entrance pupil. For an opticalsystem of a fixed etendue such as a projector SLM, associated lenssystems, and auxiliary optical components, the etendue of the lightsource should be lower than or equal to the etendue of the opticalsystem in order to efficiently illuminate the optical system withoutvignetting. Additional advantages may be gained by using an even lowersource etendue. Low source etendue means that the angle of incidence issmaller, especially for rays that are near the edge of the beam. A lowangle of incidence means that certain optical components may besimplified or may operate more effectively. For example, polarizationuniformity may be improved in LCD and LCOS SLMs, skew ray plates may notbe necessary, PBSs and polarization filters may have higher extinctionratios, multilayer interference filters may have less angle shift, andlens assemblies may be less subject to optical aberrations.

Prisms and beamsplitters are used in projectors and other opticalsystems to control the path of light beams. DBSs split or combinewavelength bands of light that form various colors and are usuallyconstructed from interference coatings on flat substrates or prismsurfaces. PBSs split or combine different polarizations of light and maybe constructed from interference coatings, prisms, or by othertechniques such as wire grids. Philips prisms consist of three subprismswith DBSs on two of the internal faces. TIR prisms have an air gapinside that makes total internal reflection when the incidence angle ofthe beam is greater than the critical angle. X-prisms consist of 4subprisms assembled into a cube such that the internal surfaces haveDBSs along both diagonal faces. Depending on their roles in the lightpath, prisms and beamsplitters may act as beam separators, beamcombiners, or both at the same time.

SLMs may be one, two, or three-dimensional. In each case, an SLMprocesses an incoming beam of light to produce an outgoing beam of lightwhich has pixels formed in a two-dimensional array. A one-dimensionalSLM has a single pixel which is scanned in two directions to form atwo-dimensional image. A one-dimensional SLM has pixels arranged in aone-dimensional line segment which is scanned in one direction to form atwo-dimensional image. A two-dimensional SLM has pixels arranged in atwo-dimensional shape such as a rectangle.

A mixing rod is used to make a light beam more spatially uniform and toform the beam into a specific cross-section, such as rectangular, sothat the beams can better match the shape of an SLM. A mixing rod may beconstructed from a solid rectangular parallelepiped where total internalreflection guides the rays of light inside to make multiple bounceswithin the mixing rod. In the case of dual illumination, there are twomixing rods, and a thin air gap may be used to keep the light withineach rod while keeping the rods as close as possible. If the lightsources are linearly polarized, orthogonal orientation of the mixingrods relative to the polarization state of the light will maintain thelinear polarization state of the light sources. If circular polarizationis desired at the output of the projectors, a quarter-wave rotationplate may be used to convert linear polarization to circularpolarization. Alternatively, instead of mixing rods, other types of beamhomogenizers may be used such as fly's eye lenses or diffusers.

Anamorphic lenses expand or compress one axis relative to the other,orthogonal axis. For example, an anamorphic lens may be used to compressthe horizontal axis relative to the vertical axis, so that the 4:1aspect ratios of the images in FIG. 32 become 2:1 aspect ratios. The useof an anamorphic lens allows substantially all of the pixels of SLM 3200to be used for imaging so that there are few or no un-used pixels. Asmall number of un-used pixels may surround the images as guard bands ifnecessary to allow for alignment tolerances.

Projection lens systems such as fifth lens system 2495 in FIG. 24, fifthlens system 2582 in FIG. 25, and fifth lens system 2692 in FIG. 26 mayconsist of many individual lens elements that are combined into one lenssystem designed to project a large image onto a screen that is locatedmany meters away from the projector. Functions such as image shifting,zooming, focusing, and other image control features may be built in theprojection lens system. FIGS. 24 through 26 show a second beam combinerand one projection lens system, but alternatively, two projection lenssystems may be used, one for each image. A second beam combiner is notnecessary if two projection lens systems are used, but a beam separatormay be required to increase the spacing between the two beams so thatthe beams can pass through the two projection lens systems.

Dual illumination of a projector is advantageous because light outputmay be increased relative to designs that use only one light source.This is particularly important for 3D projection systems that are oftenoperated below desired brightness levels. Also, the light is efficientlyused in a dual illumination system because the light is directed only tothe pixels that form the images, and does not illuminate un-used pixels.In the configurations of FIGS. 32 and 33, a double benefit is that allthe light is used and also all the pixels are used.

In one example of dual illumination, a wider gamut can be obtained byusing more than three primary colors where the colors come from morethan one light source. Red, green, and blue, may be generated by onelight source whereas yellow (or yellow and cyan) may be generated byanother light source. The two light sources may illuminate separateparts of an SLM or may overlap to illuminate the same part of the SLM.

In another example of dual illumination, an SLM may be illuminated withdifferent wavelengths of the same primary color in order to reducespeckle. The checkerboard pattern of FIG. 33 may be illuminated suchthat the pixels that are cross-hatched process one wavelength of light,and the pixels that are not cross-hatched process another wavelength oflight. The two wavelengths of light may be generated by two separatelight sources, or may be generated by one light source with two outputwavelengths. In the case where most of the speckle results from thegreen band, only the green band need be broken into two sub-bands tosignificantly reduce visible speckle.

Other optical systems include those with more than two light sourceswhich may be utilized to illuminate two or more parts of each SLM,optical systems that are not imaging such as laser-beam spatial-shapingsystems, optical systems that include non-visible light such asultraviolet or infrared radiation, optical systems that use infraredradiation to simulate night-vision scenes, and optical systems that useinexpensive SLMs with resolution of 2K or less that are subdivided intomore than one part.

In another aspect of the optical system shown in FIG. 1, novel assemblymethods may be used to align and fasten the components of the opticalsystem. In particular, the optical components of first laser lightsource 102, second laser light source 116, and third laser light source120 need precise alignment and stable positioning over the operatinglifetime of the optical system. Specific embodiments are described inthe following paragraphs but are not meant to be limiting in any way.

The conventional method of assembling and aligning optical devices maybe characterized as serial alignment. In this method, each opticalcomponent is placed into its approximate final location, and alignmentis a sequential process where each optical component is aligned usingthe beam of light of the final device. Each component's position isadjusted in as many as six degrees of freedom to find a local optimumfor that component, and then the process is repeated for every componentuntil a satisfactory overall alignment is found. This is an iterativeprocess where the local optimums depend upon each other, so eachcomponent may need to undergo the alignment process multiple times.

As an example, consider the alignment of a laser system which may havevarious optical components such as minors, lenses, optical gain stages,prisms, filters, polarization control elements, wavelength doublers,taps, and other optical components. Starting at the beginning of thelight generation path, each optical component is aligned to a localmaximum one by one in turn. If a proper combination of componentpositioning cannot be found by the time the last component is beingaligned, prior components must be readjusted. There is no deterministicmethod to find which components must be readjusted or by how much, so ahigh level of skill and experience is required to decide which partsshould be readjusted and by how much. This process is very timeconsuming and has no guarantee of success at the end.

As an aid to proper positioning of each optical component, kinematicmounts may be used to place optical components into a fixed location.Kinematic mounts consist of plates with locating features, such as ballsand rollers, which exactly constrain the six degrees of freedom suchthat an optical component is held in a unique position without wobble.The optical component combined with one side of the kinematic mountforms an optical module. The mating side of the kinematic mount is heldby a base plate. When the optical module is removed from the base plateand then put back onto the base plate, it will go back into the sameposition with high repeatability.

Referring now to the drawings, FIG. 36 shows a method of assembly foroptical components. In step 3600, kinematic mounts of an optical moduleare mated to the matching kinematic mounts of a base plate. In step3602, the optical module is held to the base plate by magnets or otherholding mechanism. In step 3604, the module is aligned with the beam ofthe final optical device which is called the actual beam. In step 3606,steps 3600 through 3604 are repeated for each optical module in theoptical system. In step 3608, the overall optical system is checked forproper functionality. If it is properly functional, step 3610 shows theassembly is finished. If not, step 3612 is to realign an optical modulewith the actual beam and return to step 3608 to check systemfunctionality. Steps 3608 and 3612 are repeated until the system isproperly functional.

FIG. 37 shows a method of assembly using pre-alignment with a referencebeam rather than the actual beam. In step 3700, an optical module isaligned to a reference beam outside of the final optical device. In step3702, the kinematic mounts of the optical module are mated to thematching kinematic mounts of a base plate. In step 3704, the opticalmodule is held to the base plate by magnets or other holding mechanism.In step 3706, steps 3700 through 3704 are repeated for each opticalmodule in the optical system. In step 3708, the overall optical systemis checked for proper functionality. If it is properly functional, step3712 shows the assembly is finished. If not, step 3710 is to adjust analignment module with the actual beam. After the single adjustment instep 3710, the overall optical alignment of the system is finished asshown in step 3712.

FIG. 38 shows a method of assembly using pre-alignment with a referencebeam, a chassis plate, and removal of the base plates. In step 3800, anoptical module is aligned to a reference beam outside of the finaloptical device. In step 3802, the kinematic mounts of the optical moduleare mated to the matching kinematic mounts of a base plate. In step3804, the optical module is held to the base plate by magnets or otherholding mechanism. In step 3806, steps 3800 through 3804 are repeatedfor each optical module in the optical system. In step 3808, the overalloptical system is checked for proper functionality. If it is properlyfunctional, step 3812 is to fasten the optical modules to a chassisplate. If it is not properly functional, step 3810 is to adjust analignment module with the actual beam. After the single adjustment instep 3810, the overall optical alignment of the system is finished andin step 3812, the optical modules are fastened to a chassis plate. Instep 3814, the base plates are removed and the assembly is finished asshown in step 3816. Alternately, the base plates may not be removed sothat they are present in the final assembly.

FIG. 39A shows a top view of base plate 3910 and FIG. 39B shows a sideview of base plate 3910. The purpose of the base plate is to positionand hold the optical module with kinematic mounts. Bottom plate 3900 hascavities 3902 which hold rollers 3904. Magnet 3906 is attached to bottomplate 3900.

FIG. 40A shows a side view of optical module 4020 and FIG. 40B shows abottom view of optical module 4020. Module plate 4000 has balls 4002mounted in its bottom surface. Optical component 4010 is attached toupper surface 4008 of module plate 4000. Light beam 4012 passes throughoptical component 4010. Magnet 4006 is attached to module plate 4000.

FIG. 41 shows a side view of optical module 4020 and base plate 3910mated together with kinematic mounting. Balls 4002 fit against rollers3904 to determine a unique kinematic mounting position for opticalmodule 4020. Magnets 3906 and 4006 come close together but do not touch,in order to hold together optical module 4020 and base plate 3910.

FIG. 42 shows a side view of optical module 4020 and base plate 3910mated together with kinematic mounting and chassis plate 4200 attachedto the optical module. After chassis plate 4200 is attached to theoptical module, base plate 3910 may be removed without disturbing thealignment of the optical module.

FIG. 43 shows six optical modules attached to chassis plate 4300. Eachoptical module has an optical component 4304, 4306, 4310, 4312, or 4314,and a module base plate 4302. Beam of light 4320 passes through theoptical components 4304, 4306, 4310, 4312, and 4314. In this example,optical components 4304, 4308, 4310, and 4314 are beamsplitter and beamcombining optical elements. Optical components 4306 and 4312 may be anyoptical elements that act on beam 4320 such as a lenses, gain stages,prisms, filters, polarizers, wavelength doublers, taps, etc. The ballsand magnets of the optical modules are not shown in FIG. 43. Morecomplex systems may be built using additional optical components inaddition to those shown in FIG. 43.

FIG. 44 shows an assembly method using an alignment plate. In thismethod, an alignment plate forms a template that is used to alignkinematic rollers on a holding plate. In step 4400, the alignment plateis placed on the holding plate. In step 4402, rollers and holding blocksare placed into holes in the alignment plate. In step 4404, the holdingblocks are fastened to the holding plate to hold the rollers in theproper position as determined by the alignment plate. In step 4406, therollers are fastened to the holding plate. In step 4408, the alignmentplate is removed. In step 4410, optical modules are mated to the rollerson the holding plate in the kinematic positions determined by therollers.

In FIG. 45A, a top view of an alignment plate and a holding plate areshown which may be used for the assembly method described in FIG. 44.Alignment plate 4502 has holes 4504. FIG. 45B shows the correspondingside view of the alignment plate. Alignment plate 4502 is placed onholding plate 4500. Rollers 4508 and holding blocks 4506 are placed inrectangular holes 4504. Holding blocks 4506 are pressed against rollers4508 which are located against the sides of holes 4504. Holding blocks4506 are fastened to holding plate 4500 to hold rollers 4508 in theproper position which is determined by the sides of holes 4504 inalignment plate 4502. The reference surfaces of alignment plate 4502include the sides of holes 4504 and the bottom of alignment plate 4502.The reference surfaces may be formed by a high tolerance machiningmethod such as electrical discharge machining so that tolerances can bekept on the order of a few micrometers. Holding plate 4500 may bemanufactured by an inexpensive method such as casting without furtherhigh-precision machining steps.

The method shown in FIG. 37 results in an optical system alignment thatis much quicker and easier to accomplish than the method shown in FIG.36. In fact, the method shown in FIG. 36 may not converge to afunctioning optical system for a fixed set of optical components. Forthe method in FIG. 37, no special skill is required for the alignment ofthe optical system because no iterations are required where guessing isnecessary to figure out which optical elements to adjust and by howmuch. Also, the method in FIG. 36 generally requires large adjustmentranges on the order of 5 mm to cover the entire span that might berequired. Finding the best alignment position in this large range istime-consuming. The method in FIG. 37 requires much smaller adjustmentranges on the order of 200 micrometers and finding the best alignmentposition in the small range is very quick. Another advantage of themethod in FIG. 37 is that by changing pre-aligned optical modules, thesystem may be easily repaired. Also each optical module may be made andaligned in a different location or at a different company and thenshipped to one location for assembly with only minimal final adjustment.

An advantage of the method shown in FIG. 38 and the device shown in FIG.42 is that the base plates may be removed and the relatively inexpensivechassis plate may be substituted instead for the final assembly. Oncethe optical modules are fastened to the chassis plate, the base platesare no longer required to hold the optical modules in the properposition.

An advantage of the method shown in FIG. 44 and the device shown inFIGS. 45A and 45B is that the relatively high-tolerance and expensivealignment plate may be reused many times to align the optical modules.Once the rollers are held in the proper position, the alignment platemay be removed. The alignment plate is in effect “printing” thealignment positions onto the less expensive parts such as the holdingplate.

FIG. 46A shows a schematic diagram of the alignment error bars of eachoptical component for the method of FIG. 36. FIG. 46B shows thealignment error bars of each optical component for the method of FIG.37. The vertical axis represents the amount of error from nominal beampath 4600. The horizontal axis represents travel through the opticalsystem. Note how ranges 4606 of FIG. 45B are smaller and close tonominal position 4600 compared to ranges 4602 in FIG. 45A. The result isthat aligned beam 4608 is closer to nominal beam 4600 than aligned beam4604. Whereas in the method of FIG. 36, the alignment objective is toalign each module to the previous one, in the method of FIG. 37, thealignment objective is to make the beam of light travel in the correctlocation. Some optical components may require high tolerances such asapertures, and some may require lower tolerances such as lenses, but forthe purpose of FIGS. 46A and 46B all tolerances are shown as equal.

In addition to the kinematic mounting method with three balls and threegroves as described in FIGS. 39 through 41, other kinematic mountingmethods may be used. Six points of contact are required to exactlydetermine the six degrees of freedom (three linear and three angular) ofeach optical component. As an example of a different kinematic mountingmethod, a flat-groove-pyramid kinematic mount has one ball that contactsa pyramid in three points, one ball that contacts a groove in twopoints, and one ball that contacts a flat at one point.

Holding methods are intended for temporary positioning during assembly.Various methods may be used to hold together the kinematic mounts of theoptical module and the base plate. Magnets are shown in FIGS. 39 through41. Other methods may include bolts, springs, and flexure blades. Thesemethods generally allow the optical module to be easily removed from thebase plate.

Flexure blades are thin strips of metal that are strong in twodirections and flexible in one direction. Two orthogonal flexure bladesfix an optical component in all six degrees of freedom.

Fastening methods are intended for permanent or semi-permanentattachment. Fastening methods include adhesives such as two-part epoxiesor ultraviolet-cure epoxies, soldering, brazing, and welding. If anadhesive is used, the thickness of the bond line should be minimal sothat there is no possibility of the glue expanding or contracting overtime or with temperature changes. In the case of repair or rework, thefastening may be removed and a new component installed, aligned, andfastened again in place of the previous component. Robotic assemblytechniques may be used if advantageous for cost, cleanliness,throughput, or other reasons.

When fastening methods are used rather than optical adjustment mountssuch as angular or linear positioners, there are no adjustments to driftduring the lifetime of the optical system. Also, there is a cost savingswhen no angular or linear positioners are required in the final product.

Kinematic mounts are not capable of withstanding large shocks such asthose that might occur during transportation. Even when the balls androllers are made of hard materials such as silicon nitride or hardenedstainless steel, the point contacts of the curved surfaces tend to getflattened if there is too much pressure applied. Kinematic mounts madewith conventional materials can withstand up to approximately 90 newtons(20 pounds) of force per ball before damage becomes a problem. Onceflattened, the kinematic mount may have unacceptable wobble. Anadvantage of using the chassis as shown in FIG. 42 is that the kinematicmounts are removed so the entire assembly is ruggedized to withstandtransportation or other environmental shock and vibration.

Using the method of FIG. 37 or 38, the entire optical system may beshipped to its final destination of usage in the unaligned state andthen easily aligned on site. During shipping, the mating parts of thekinematic mounts may be separated completely or they may be separatedonly slightly to avoid damage from shock or vibration. A cam mechanismmay be used to lift the optical modules from the base plates duringshipping and then reseat them when the optical system is ready to beused.

The balls, rollers, and magnets in FIGS. 39A, 39B, 40A, 40B, 41, and 42may be attached to their respective plates using epoxy. The minimumthickness of the plates to keep optical tolerances may be in the rangeof 5 mm thick to 15 mm thick depending on the size of the part. Manyoptical parts for laser applications must be controlled to a toleranceof a few arc-seconds in two angular directions and a few micrometers intwo linear dimensions.

The segments of the beam path may form a rectilinear shape (meaning allangles between the segments are 180 degrees or 90 degrees) to reduce thealignment changes that result from thermal expansion and contraction ofthe entire optical assembly. All the vertical supporting structures maybe constructed from the same material and all the horizontal supportingstructures may be from the same material which may be different than thematerial of the vertical supporting structures. For example, all thevertical supporting structures may be constructed from stainless steeland all the horizontal supporting structures may be constructed fromaluminum. Since there is generally more material in the horizontalsupporting structures, a lightweight material such as aluminum may beused for the horizontal supporting structures. The base plate may be theprimary horizontal supporting structure and the primary verticalsupporting structures may be in the optical modules.

The method of FIG. 36 may result in a situation where the optical systemmay be functioning but close to the non-functioning state if itexperiences thermal stress, aging, or other natural changes. Forlow-duty cycle systems such as medical laser applications, this may besufficient, but for consistent operation for many hours per day, themethod of FIG. 37 results in a more reliable optical system.

The object of the method in FIG. 37 is to position the path of the lightbeam into the proper location. The position of the optical modulesdepends on the proper position of the path of the light beam. Lowtolerance parts may be used for the optical modules because the partsare aligned using the reference beam. The optical system may be alignedto reach the global maximum. In the method of FIG. 36, the oppositeapproach is used; the optical modules are positioned into desiredlocations and the position of the light beam depends on the position ofeach optical module. High tolerance parts are generally used to helpmake the alignment go quicker, but the optical system may only bealigned to a local maximum. The global maximum may not be reached.

In laser optical systems, collimated light beams generally have twoangular degrees of freedom, azimuth and elevation, and two lineardegrees of freedom, x and y, that must be adjusted to high tolerances.The remaining two degrees of freedom, roll and z, generally do not needto be adjusted. The z-direction is defined as the linear direction alongthe path of the beam.

In another aspect of the optical system shown in FIG. 1, a supportstructure with compartments and stackable layers may be used for theoptical assembly of first laser light source 102, and if used, secondlaser light source 116 and third laser light source 120. The stackedcompartmented support structure makes a stable optical assembly with lowweight. Specific embodiments are described in the following paragraphsbut are not meant to be limiting in any way.

The conventional method of assembling optical systems utilizes a rigid,flat, support structure that may be a solid block of metal or, to reduceweight, a honeycomb structure with a flat top. An optical systemconsists of a number of optical components. Each component needs to bealigned and fastened into the proper position for the entire system tofunction correctly. The optical components must be held in place overtemperature changes and as the structure ages. There is a trade-offbetween rigidity and weight, because more material in the structure isgenerally necessary to make it more rigid.

FIG. 47A shows a top view of an optical assembly built on a flat supportstructure and FIG. 47B shows a side view of the same assembly. Theoptical assembly consists of optical modules which each have one or moreoptical components and the supporting structure. First light beamsegment 4708 enters first optical module 4710 and is processed by firstoptical module 4710 to form second light beam segment 4712. Second lightbeam segment 4712 enters second optical module 4714 and is processed bysecond optical module 4714 to form third light beam segment 4716. Thirdlight beam segment 4716 enters third optical module 4718 and isprocessed by third optical module 4718 to form fourth light beam segment4720. Fourth light beam segment 4720 enters fourth optical module 4722and is processed by fourth optical module 4722 to form fifth light beamsegment 4724. Fifth light beam segment 4724 enters fifth optical module4726 and is processed by fifth optical module 4726 to form sixth lightbeam segment 4728. The optical modules are mounted to flat supportstructure 4700. Each optical module performs an optical function such asfocusing the light beam, amplifying the light beam, reflecting the lightbeam, or changing the wavelength of the light beam with nonlinearoptics. The optical modules may be simple, consisting of singlecomponents such as lenses, minors, or filters, or the optical modulesmay be complex, consisting of multiple components in each optical modulethat form lasers, gain stages, second harmonic generators, or opticalparametric oscillators. The optical modules may be mounted to the flatsupport structure using kinematic mounts. The light beams in FIG. 47 maybe laser beams or may be other light beams such as incoherent beamsformed by lenses.

FIG. 48A shows a top view of a compartmented support structure and FIG.48B shows a side view of the same structure. Compartmented supportstructure 4800 has first compartment 4802, second compartment 4804, andthird compartment 4806. First hole 4830 is a feedthrough hole thatenables a beam of light to enter first compartment 4802. Second hole4832 enables a beam of light to travel between first compartment 4802and second compartment 4804. Third hole 4834 enables a beam of light totravel from second compartment 4804 to third compartment 4806.Compartmented support structure 4800 has sufficient height so thatoptical modules may be mounted within its compartments. More or fewercompartments may be included and the compartments may have variousshapes depending on how many optical modules are desired insidecompartmented support structure 4800, the shapes of the optical modules,and the desired layout of the optical modules. Compartmented supportstructure 4800 may be constructed of one solid piece of material, or itmay be constructed from separate pieces of material that are attachedtogether.

FIG. 48C shows a top view of a compartmented support structure with afeedthrough hole in a different location compared to FIG. 48A. FIG. 48Dis a side view of the same structure shown in FIG. 48C. Compartmentedsupport structure 4810 has first compartment 4812, second compartment4814, and third compartment 4816. First hole 4840 is a feedthrough holethat enables a beam of light to enter first compartment 4812. Secondhole 4842 enables a beam of light to travel between first compartment4812 and second compartment 4814. Third hole 4844 enables a beam oflight to travel from second compartment 4814 to third compartment 4816.In FIG. 48A, the light circulates through the compartments clockwise,whereas in FIG. 48C, the light circulates through the compartmentscounter-clockwise.

FIG. 49A shows a top view of an optical assembly built with acompartmented support structure and FIG. 49B shows a side view of thesame assembly. FIGS. 49A and 49B show the optical modules of FIGS. 47Aand 47B mounted inside the compartmented support structure of FIGS. 48Aand 48B. First light beam segment 4908 travels through first hole 4930to enter optical module 4910 in first compartment 4902 and is processedby first optical module 4910 to form second light beam segment 4912.Second light beam segment 4912 travels through second hole 4932 to entersecond optical module 4914 in second compartment 4904 and is processedby second optical module 4914 to form third light beam segment 4916.Third light beam segment 4916 enters third optical module 4918 incompartment 4904 and is processed by third optical module 4918 to formfourth light beam segment 4920. Fourth light beam segment 4920 travelsthrough third hole 4934 to enter fourth optical module 4922 incompartment 4906 and is processed by fourth optical module 4922 to formfifth light beam segment 4924. Fifth light beam segment 4924 entersfifth optical module 4926 in compartment 4906 and is processed by fifthoptical module 4926 to form sixth light beam segment 4928. The opticalmodules are mounted to compartmented support structure 4900. Thefunctions of the light beam segments and optical modules may be the sameas in FIGS. 1A and 1B. First light beam segment 4908 vertically entersfirst optical module 4910 from the bottom of compartmented supportstructure 4900. Sixth light beam segment 4928 vertically exits fifthoptical module 4926 from the top of compartmented support structure4900. The dots representing first light beam segment 4908 and sixthlight beam segment 4928 in FIG. 49A signify that the light beams aretraveling towards the viewer. The cross representing third light beamsegment 4916 in FIG. 49B signifies that the light beam is traveling awayfrom the viewer.

FIG. 50A shows a side view of a stacked compartmented support structureand FIG. 50B shows a side view of the same assembly. The stackedcompartmented support structure may be populated by optical modules toform a stacked optical assembly. The stacked compartmented supportstructure shown in FIGS. 50A and 50B is composed of five compartmentedsupport structures of the type shown in FIGS. 48A, 48B, 48C, and 48D.The five compartmented support structures, first compartmented supportstructure 5000, second compartmented support structure 5002, thirdcompartmented support structure 5004, fourth compartmented supportstructure 5006, and fifth compartmented support structure 5008, arestacked vertically one on top of the other with lid 5010 on top of fifthcompartmented support structure 5008. First compartmented supportstructure 5000 is of the type shown in FIGS. 48A and 48B except that ithas no feedthrough hole. Third compartmented support structure 5004 andfifth compartmented support structure 5008 are of the type shown inFIGS. 48A and 48B. Second compartmented support structure 5002 andfourth compartmented support structure 5006 are of the type shown inFIGS. 48C and 48D. Optical modules and beams of light are not shown inFIGS. 50A and 50B, but may be installed as shown in FIGS. 49A and 49B.The initial light beam is generated in first compartment 5020 of firstcompartmented support structure 5000 and travels through holes 5060,5064, 5068, 5070, 5072, 5076, 5080, 5082, 5084, 5086, 5088, and otherholes not shown as it goes through compartments 5022, 5026, 5030, 5032,5034, 5038, 5042, 5044, 5046, 5048 and other compartments not shown,rising through the stack to exit from final hole 5088 in lid 5010.

FIG. 51 shows an assembly method for a stacked compartmented supportstructure assembled in parallel. In this method, each layer ofcompartmented support structure is assembled in parallel and then allthe compartmented support structures are stacked. In step 5100, opticalmodules are mounted in a compartmented support structure. In step 5102,the optical modules are aligned in the compartmented support structure.Reference beams may be used for this alignment. In step 5104, steps 5100and 5102 are repeated for all of the compartmented support structuresthat will form the stacked compartmented support structure. In step5106, the compartmented support structures are assembled into a verticalstack. In step 5108, final alignment is performed if necessary. Thefinal alignment points of interior optical modules may be reachedthrough alignment holes in the stacked compartmented support structure.In step 5110, the lid is attached to the stacked compartmented supportstructure.

FIG. 52 shows an assembly method for a stacked compartmented supportstructure assembled in series. In this method, each layer ofcompartmented support structure is assembled in series and assembledinto the stacked compartmented support structure before the next layerof compartmented support structure is assembled. In step 5200, opticalmodules are mounted in the first compartmented support structure whichwill form the bottom of the stacked compartmented support structure. Instep 5202, the optical modules are aligned in the first compartmentedsupport structure. Reference beams may be used for this alignment. Instep 5204, an additional compartmented support structure is attached tothe top of the previous compartmented support structure. In step 5206,optical modules are mounted in the additional compartmented supportstructure. In step 5208, optical modules are aligned in the additionalcompartmented support structure. In step 5210, steps 5204, 5206, and5208 are repeated for all the additional compartmented supportstructures that will form the stacked compartmented support structure.In step 5212, the lid is attached to the stacked compartmented supportstructure.

Kinematic mounting techniques may be used for mounting the opticalmodules to the compartmented support structures. Kinematic mountingtechniques may also be used for stacking each layer of compartmentedsupport structure to the underlying layer. The stacked compartmentedsupport structure may be held together by bolts that go through theentire assembly from the bottom of the bottom compartmented supportstructure to the top of the top compartmented support structure or theremay be bolts that attach each compartmented support structure only toits underlying compartmented support structure. Kinematic mountingtechniques are helpful to enable quicker assembly and to allow opticalmodules or an entire layer of compartmented support structure to bereplaced in case of malfunction. A kinematic mount may be located in thecompartmented support structure, on the exterior surface of an opticalmodule, on the exterior surface of the compartmented support structure,or on the mating surfaces between the compartmented support structures.

The vertical optical beams and vertical feedthrough holes shown in FIGS.50A and 50B pass through points on only one side of the stackedcompartmented support structure. Alternatively, some vertical opticalbeams may pass through one side of the support structure and somevertical optical beams may pass through the other side of the supportstructure. Using both sides of the support structure may increase theruggedness of the assembly. The transfer of light between compartmentsor between layers of compartmented support structure may be performed byconventional relay optics consisting of lenses which are designed togather light beams and transmit them over the appropriate distanceswithout significant losses.

In FIGS. 47A, 47B, 48A, 48B, 48C, 48D, 49A, 49B, 50A, and 50B, certainmechanical and optical features are shown, but supporting electricalfeatures are not shown. Appropriate wires and electrical feedthroughsare required for electrically active optical modules such as those thatcontain semiconductor lasers, gain stages, gas discharges, or otherelectrical components.

The compartmented support structures may be composed of any stiff andoptically opaque material. Metals such as aluminum and stainless steelmay be used. If weight is a primary consideration, lighter materialssuch as high-strength plastic or graphite composite materials may beused. If optical transparency is desired in certain regions of thecompartmented support structure, transparent glass or crystal windowmaterial may be used in those regions. Alternately, some regions of thecompartmented support structure may be left open.

Although FIGS. 48A, 48B, 48C, 48D, 49A, 49B, 50A, and 50B show threecompartments per compartmented support structure, one, two, or morecompartments may be formed in each compartmented support structure. Eachcompartment isolates the optical modules in that compartment. Straylight is therefore contained within each compartment. Black-paintedsurfaces or the use of black structural materials may be used to absorbstray light. The layout of the compartments may be of any design to fitthe desired optical modules inside. Each compartmented support structurein the stacked optical assembly may have a different layout of itscompartments.

Although FIGS. 50A and 50B show five compartmented support structurelayers in the stacked optical assembly, there may be any number ofcompartmented support structure layers in the stacked optical assembly.If many optical modules are required to build a complex optical system,more compartmented support structure layers may be added to the designto accommodate the increased complexity. Alternately, more compartmentsmay be added to the design of each layer. The overall shape of thestacked compartmented support structure may be a rectangularparallelepiped as shown in FIGS. 50A and 50B, or it may be another shapesuch as cylindrical, pyramidal, irregular polygonal or any otherthree-dimensional shape.

The wall thickness of the compartmented support structure should besufficient to ensure stable positioning of the optical modules inside.The wall thickness may be in the range of 2 mm to 15 mm or in the rangeof 5 mm to 10 mm. High tolerances may be required for the opticalmodules inside the compartmented support structure. Optical modules ofprecision systems such as lasers may require positioning with lineartolerances on the order of three micrometers and angular tolerances onthe order of three arc-seconds.

The layout of optical modules within compartments may be determined bymany factors such as the required order of the optical modules, thermalconsiderations, and electrical considerations. Some of the modules inthe compartmented support structure may not have optical components asthey may be supporting electrical components or serve other non-opticalfunctions.

As an example of an optical system using a stacked compartmented supportstructure, a multicolor laser system may have services such as watercooling and electrical power supplies in the bottom (first)compartmented support structure, Q-switched laser oscillators in thenext (second) compartmented support structure, optical gain modules inthe next (third) compartmented support structure, another layer ofoptical gain modules in the next (fourth) compartmented supportstructure, OPOs in the top (fifth) compartmented support structure, anda lid on the top compartmented support structure. In this example, eachcompartmented support structure may be 3.5 cm high and the entire sizeof the stacked compartmented support structure may be 48 cm long by 43cm wide by 18 cm high. The second layer of gain modules may not benecessary if the desired output power of the multicolor laser system maybe attained with a single layer of gain modules.

There are a number of advantages to using the stacked compartmentedsupport structure relative to using the flat support structure. Thestacked compartmented support structure is stiffer, has less weight,smaller linear dimensions, less overall volume, less surface area, andis more rugged for the same number of optical modules. The structuralpieces may be made thinner to obtain equivalent stiffness and opticalpositioning tolerances.

For optical systems that include lasers, there are also improvements inlaser safety during assembly because each layer of the compartmentedsupport structure may be sealed and eye safe in serially assembledsections when the next layer is stacked. The entire system is also lasersafe after final assembly. Light leakage between compartments and layerscan also be controlled so that light leakage does not adversely impactthe optical function of the system. Air flow may also be controlledbetween compartments and layers if desired. The compartmented supportstructure is also more modular and interchangeable allowing easy repairof individually tested modules and known working subsystems. Service mayconsist of taking apart and replacing a layer of compartmented supportstructure without disturbing the alignment of the other layers.

The compartmented support structure may be adapted to support opticalmodules by using kinematic mounts, by the layout of the compartments andholes between the compartments, by designing with stiffness sufficientfor high tolerance optical positioning, by separating or absorbing lightin each compartment, or by any other means that enable an optical systemto function properly when mounted in the compartmented supportstructure.

In another aspect of the optical system shown in FIG. 1, a stabilizedOPO 104 may be constructed by using collimated beams of light betweenoptical modules in the OPO. The OPO may be quicker and easier to alignand stay in alignment for a longer period of time when using collimatedbeams of light. Specific embodiments are described in the followingparagraphs but are not meant to be limiting in any way.

A detailed description of an optical system generating three colors ofvisible light using an OPO may be found in U.S. Pat. No. 5,740,190, thecomplete disclosure of which is incorporated herein by reference.Starting with a visible pump beam, an OPO makes an infrared signal beamand an infrared idler beam. By choosing a specific temperature and othercharacteristics of the OPO crystal, the signal beam and idler beam maybe tuned as desired. For example, if the pump beam has a wavelength of523.5 nm, the two additional beams created may be at 898 nm and 1252 nmwhich are in the infrared region. By using two second-harmonicgenerators, the 898 nm light can be frequency doubled to 449 nm lightwhich is blue, and the 1252 nm light can be frequency doubled to 626 nmwhich is red. This produces red, green, and blue colors of visible lightthat may be used by a digital image projection system. By choosing adifferent temperature of the OPO, the infrared wavelengths may be 904 nmand 1242 nm which can be frequency doubled to visible wavelengths of 452nm and 621 nm which fit the color gamut specified for digital cinema.Other wavelengths within the blue or red range may give acceptable colorperformance for digital cinema projection or other types of digitalimage projection. Blue wavelengths may be in the range of 430 nm to 480nm and red wavelengths may be in the range of 600 nm to 680 nm. The pumpbeam may be in the middle of the green region of visible light whichextends from 510 nm to 550 nm. The pump laser may be a Nd:YLF laserwhich emits light at 1047 nm that can be frequency doubled to 523.5 nm,a Nd:YAP laser which emits light at 1079.5 nm that can be doubled to 540nm, or other lasers with other wavelengths.

FIG. 53 shows an optical system with an optical parametric oscillatorthat generates two additional colors of light from one pump beam. Pumpbeam segment 5302 passes through first SWP filter 5304 and enters OPO5306 which converts part of pump beam segment 5302 into colocatedsignal, idler, and remaining pump beam segment 5308. Colocated signal,idler, and remaining pump beam segment 5308 passes to second SWP filter5310 which reflects first idler beam segment 5312 and passes colocatedsignal and remaining pump beam segment 5334. First idler beam segment5312 is focused by first lens 5314 to form second idler beam segment5316. Second idler beam segment 5316 reflects from first mirror 5318 toform third idler beam segment 5320. Third idler beam segment 5320 passesinto first SHG 5322. First SHG 5322 converts part of third idler beamsegment 5320 to form colocated second harmonic and remaining idler beamsegment 5324. Colocated second harmonic and remaining idler beam segment5324 passes to third SWP filter 5326 which reflects fourth idler beamsegment 5328 and passes first second-harmonic beam segment 5340. Fourthidler beam segment 5328 is focused by second lens 5330 to form fifthidler beam segment 5332. Fifth idler beam segment 5332 reflects fromfirst SWP filter 5304 to join together with pump beam segment 5302.

First SWP filter 5304, OPO 5306, second SWP filter 5310, first lens5314, first mirror 5318, first SHG 5322, third SWP filter 5326, andsecond lens 5330 form recirculating optical subsystem 5300.Recirculating optical subsystem 5300 recirculates the idler beam so itpasses multiple times through OPO 5306 and first SHG 5322. The opticalinput to recirculating optical subsystem 5300 is pump beam 5302 and theoptical outputs are colocated signal and remaining pump beam segment5334 and first second-harmonic beam segment 5340.

After leaving recirculating optical subsystem 5300, colocated signal andremaining pump beam segment 5334 passes to fourth SWP filter 5336 whichreflects first signal beam segment 5350 and passes remaining pump beamsegment 5338. First signal beam segment 5350 reflects from second mirror5352 to form second signal beam segment 5354. Second signal beam segment5354 is focused by third lens 5356 to form third signal beam segment5358. Third signal beam segment 5358 passes into second SHG 5360 whichconsists of first SHG crystal 5362 and second SHG crystal 5364. FirstSHG crystal 5362 and second SHG crystal 5364 form a walk-off SHG systemwhich converts part of third signal beam 5358 to form colocated secondharmonic and remaining signal beam 5366. Colocated second harmonic andsignal beam 5366 passes to LWP filter 5368 which reflects secondsecond-harmonic beam segment 5372 and passes remaining signal beam 5370.Remaining signal beam 5370 passes into beam dump 5378 and is absorbed inbeam dump 5378. Second second-harmonic beam segment 5372 reflects fromthird minor 5374 to form third second-harmonic beam segment 5376.

Fourth SWP filter 5336, second mirror 5352, third lens 5356, first SHGcrystal 5362, second SHG crystal 5364, LWP filter 5368, third minor5374, and beam dump 5378 form beam separation and conversion system5380. The optical input to beam separation and conversion system 5380 iscolocated signal and remaining pump beam segment 5334 and the opticaloutputs are remaining pump beam segment 5338 and third second-harmonicbeam segment 5376.

FIG. 54 is a schematic view of a recirculating optical subsystem withfour relay lenses rather than two relay lenses as shown in FIG. 53. Pumpbeam segment 5402 passes through first SWP filter 5404 and enters OPO5406 which converts part of pump beam segment 5402 into colocatedsignal, idler, and remaining pump beam segment 5408. Colocated signal,idler, and remaining pump beam segment 5408 passes to second SWP filter5410 which reflects first idler beam segment 5412 and passes colocatedsignal and remaining pump beam segment 5442. First idler beam segment5412 is focused by first lens 5414 to form second idler beam segment5416. Second idler beam segment 5416 is focused by second lens 5418 toform third idler beam segment 5420. Third idler beam segment 5420reflects from mirror 5422 to form fourth idler beam segment 5424. Fourthidler beam segment 5424 passes into SHG 5426. SHG 5426 converts part offourth idler beam segment 5424 to form colocated second harmonic andremaining idler beam segment 5428. Colocated second harmonic andremaining idler beam segment 5428 passes to third SWP filter 5430 whichreflects fifth idler beam segment 5432 and passes second-harmonic beamsegment 5444. Fifth idler beam segment 5432 is focused by third lens5434 to form sixth idler beam segment 5436. Sixth idler beam segment5436 is focused by fourth lens 5438 to form seventh idler beam segment5440. Seventh idler beam segment 5440 reflects from first SWP filter5404 to join together with pump beam segment 5402. First SWP filter5404, OPO 5406, second SWP filter 5410, first lens 5414, second lens5418, minor 5422, SHG 5426, third SWP filter 5430, third lens 5434, andfourth lens 5438 form recirculating optical subsystem 5400.

FIG. 55 shows additional details of the recirculating optical subsystemshown in FIG. 54. In FIG. 55, the beam extents are shown schematicallyas two lines, whereas in FIG. 53, the beams are shown schematically assingle lines. Pump beam segment 5402 is focused so that it forms beamwaist 5450 in OPO 5406. First idler beam segment 5412 is focused byfirst lens 5414 to collimated second idler beam segment 5416 which iscollimated. Second idler beam segment 5416 is focused by second lens5418 to form third idler beam segment 5420 and fourth idler beam segment5424 which forms beam waist 5452 in SHG 5426. Fifth idler beam segment5432 is focused by third lens 5434 to form sixth idler beam segment 5436which is collimated. Sixth idler beam segment 5436 is focused by fourthlens 5438 to form seventh idler beam segment 5440 which joins togetherwith pump beam segment 5402 to form beam waist 5450 in OPO 5406. Thebeam extent shown in FIG. 55 is exaggerated to more clearly show thedifference between focused and collimated.

FIG. 56 shows a non-rectilinear recirculating optical subsystem.Relative to the rectilinear recirculating optical subsystem shown inFIG. 55, the non-rectilinear recirculating optical subsystem shown inFIG. 56 has second SWP 5410 moved farther away from OPO 5406 and thirdSWP 5430 moved farther away from SWG 5426. Also first SWP 5404, secondSWP 5410, minor 5422, and third SWP 5430 are rotated approximately fivedegrees clockwise relative to FIG. 54. This produces a non-rectilinearrecirculating optical subsystem. The shape of the recirculating opticalsubsystem in FIG. 56 is a parallelogram. By changing the positions offirst SWP 5404, second SWP 5410, minor 5422, and third SWP 5430 andadjusting those four components to the appropriate angles so that thespecular reflections circulate the beams around the recirculatingoptical subsystem, various quadrilateral shapes may be produced. A crossshape may also be formed by adjusting the SWPs and minor so that thebeams cross in the middle.

FIG. 57 shows a method of generating light. In this method, twoadditional colors of light are generated from one pump beam. Includingthe remaining pump beam, the total output is three colors of light. Instep 5700, a pump beam is focused into an OPO. In step 5702, the OPOforms a signal beam and an idler beam. In step 5704, the idler beam isseparated from the signal beam and pump beam. In step 5706, the idlerbeam is focused through at least two lenses into an SHG. In step 5708,the SHG forms a second-harmonic beam from the idler beam. In step 5710,the second-harmonic beam is separated from the idler beam and thesecond-harmonic beam is output. In step 5712, the idler beam is focusedback through at least two lenses to join the pump beam and enter theOPO. In step 5714, the remaining pump beam is separated from the signalbeam and the remaining pump beam is output. In step 5716, the signalbeam is focused through a lens into an SHG. In step 5718, the SHG formsa second-harmonic beam from the signal beam. In step 5720, thesecond-harmonic beam is separated from the remaining signal beam and thesecond-harmonic beam is output. In step 5722, the remaining signal beamis directed into a beam dump that absorbs the remaining signal beam.

A detailed description of OPOs may be found in U.S. Pat. No. 5,740,190.The wavelengths of the pump, signal, and idler beam are related by thefollowing mathematical expression: 1/λ_(p)=1/λ_(s)+1/λ_(i), where λ_(p)is the wavelength of the pump beam, 1/λ_(s) is the wavelength of thesignal beam, and 1/λ_(i) is the wavelength of the idler beam. Thewavelengths also depend on various parameters of the crystal such as itssize, orientation, and temperature. Some of the requirements for highefficiency conversion include phase matching, good beam quality, andsufficiently high beam density. Q-switched lasers may be used achievesufficient beam density by using short pulses and low duty cycles. TheOPO may be an x-cut LBO crystal with propagation along the x-axis,noncritical phase matching, and temperature controlled at 159.29 degreesCelsius.

A detailed description of SHGs may be found in U.S. Pat. No. 4,019,159,the complete disclosure of which is incorporated herein by reference.SHGs use nonlinear optical processes to convert the wavelength of theoriginal light beam into a harmonic wavelength such as half the originalwavelength. This is equivalent to doubling the frequency of the lightbeam. The SHGs shown in FIGS. 53, 54, 55, and 56 may be constructed fromLBO.

Phase matching in OPOs and SHGs may be divided into two types. Type I isdefined as the condition where two input beams have the samepolarization and type II is defined at the condition where two inputbeams have orthogonal polarization. In the case where there is one inputbeam, it can be considered two input beams with the same polarization.In FIGS. 53, 54, 55, and 56, OPOs 5306 and 5406 may be of type I. InFIG. 53, first SHG 5322 may be of type II and second SHG 5360 may be oftype I. In FIGS. 54, 55, and 56, SHG 5426 may be of type II.

SWP and LWP filters may be formed by conventional methods such as thedeposition of multilayer interference filters with alternating layers ofhigh index and low index of refraction materials that are designed totransmit certain wavelengths while reflecting other wavelengths. The SWPand LWP filters shown in FIGS. 53, 54, 55, and 56 may bevacuum-deposited interference filters on flat, glass substrates.

Optical lenses are used to focus the beams into the nonlinear crystalsof the OPOs and SHGs. A narrow focal point (beam waist) is helpful toreach the high power density required for nonlinear optical processes.Additionally, in order for the recirculating optical subsystem to workefficiently, the beam must go around the recirculation path repeatingthe position of each beam waist within a variation of approximately 10%of the width of the beam waist after 10 circuits. For example, if thebeam waist is 80 micrometers wide, the idler beam must go around therecirculating optical subsystem 10 times while drifting less than 8micrometers.

The recirculating optical subsystem shown in FIGS. 54, 55, and 56 withfour lenses has a number of advantages over the recirculating opticalsubsystem shown in FIG. 53 which has only two lenses. One advantage ofthe four-lens system is that the beams which travel between the OPO andthe SHG may be collimated. This makes the collimated section insensitiveto length changes that may result from temperature changes or drift overthe lifetime of the system. By placing first SWP filter 5404 and secondSWP filter 5410 close to and in the same optical module as OPO 5406,those three components become one optical module which minimizespossible position changes in that module. In the same manner, by placingmirror 5422 and third SWP filter 5430 close to and in the same opticalmodule as SHG 5426, those three components become one optical module.Collimated beams which are insensitive to misalignment are used to coverthe relatively long distance between the OPO optical module and the SHGoptical module.

A second advantage of the four-lens system is that alignment is mucheasier because more configurations are possible solutions for opticalalignment of the recirculating optical subsystem. Two-lens systems suchas the one shown in FIG. 53 generally must be rectilinear to a highaccuracy. The four-lens system in FIGS. 54 and 55 has many possiblealignment solutions that are not rectilinear such as the parallelogramshape shown in FIG. 56.

A third advantage of the four-lens system is that the issue of keepingthe beam waist centered in the crystal can be separated from the issueof keeping the beam waist coming back into the same position each timeit travels around the recirculating system. These two issues are closelyinterrelated during the alignment of the two-lens system.

In another aspect of the optical system shown in FIG. 1, an improvedlaser gain module 106 may be constructed by using retroreflectiveminors. In addition to being easier to align, the laser gain module maymore efficient when retroreflective minors are used. Specificembodiments are described in the following paragraphs but are not meantto be limiting in any way.

A detailed description of transversely-pumped solid state lasers may befound in U.S. Pat. No. 5,774,489. When a transversely-pumped solid statelaser is utilized to amplify laser light from an external laser, thetransversely-pumped solid state laser can be considered to be a lasergain module. Optical side pumping of a gain medium in the laser gainmodule sets up a gain sheet which is used to optically amplify theexternal laser by stimulated emission in the gain sheet.

FIG. 58 shows a top view of the operation of laser gain module 5824.Legend 5820 defines the X, Y, Z, and φ directions. Angle φ is defined asa rotation in the X-Z plane. External laser 5822 produces input laserbeam segment 5800 which enters gain slab 5802. Pump lasers 5816 producepump beams 5818 which also enter gain slab 5802. Input laser beamsegment 5800 is optically amplified by an inversion population ofexcited atoms in gain slab 5802 created by pump beams 5818. Input laserbeam segment 5800 reflects off mirror 5804 to form laser beam segment5806. Laser beam segment 5806 reflects off minor 5808 to form laser beamsegment 5810. Laser beam segment 5810 reflects off minor 5804 to formlaser beam segment 5812. Laser beam segment 5812 reflects off minor 5808to form output laser beam segment 5814. The multiple laser beam segmentscrossing gain slab 5802 allow input laser beam segment 5800 to beamplified multiple times in the process of becoming output laser beamsegment 5814. Output laser beam segment 5814 exits laser gain module5824. Laser beam segments 5800, 5806, 5810, 5812, and 5814 combine toform a main laser beam which is distinct from pump beams 5818.

The number of times that the main laser beam crosses slab 5802 dependson the entrance angle of laser beam segment 5800 and the alignmentangles of mirrors 5804 and 5808. Minors 5804 and 5808 may be flat or mayhave a slight curvature. Main laser beam segments 5800, 5806, 5810,5812, and 5814 travel along paths that are substantially parallel to theZ direction. Pump beams 5818 travel along paths that are substantiallyparallel to the X direction.

FIG. 59 shows a side view of the operation of laser gain module 5824.Legend 5904 shows the X, Y, and Z directions which are rotated relativeto legend 5820 in FIG. 58. Angle θ is defined as a rotation in the Y-Zplane. Gain sheet 5902 is in gain slab 5802. Gain sheet 5902 is formedby the optical pumping of pump lasers 5816 as shown in FIG. 58. Laserbeams 5900 represent the main laser beam. Laser beams 5900 are opticallyamplified when they pass through gain sheet 5902.

FIG. 60 shows a top view of the operation of laser gain module 6024.Legend 6020 defines X, Y, Z, and φ as in FIG. 58. External laser 6022produces input laser beam segment 6000 which enters gain slab 6002. Pumplasers 6016 produce pump beams 6018 which also enter gain slab 6002.Input laser beam segment 6000 is optically amplified by an inversionpopulation of excited atoms in gain slab 6002 created by pump beams6018. A gain sheet is created which is similar to the gain sheet shownin FIG. 59. Input laser beam segment 6000 reflects off TIR prism 6004twice to form laser beam segment 6006. Laser beam segment 6006 reflectsoff TIR prism 6008 twice to form laser beam segment 6010. Laser beamsegment 6010 reflects off TIR prism 6004 twice to form laser beamsegment 6012. Laser beam segment 6012 reflects off TIR prism 6008 twiceto form output laser beam segment 6014. The multiple laser beam segmentscrossing gain slab 6002 allow input laser beam segment 6000 to beamplified multiple times in the process of becoming output laser beamsegment 6014. Output laser beam segment 6014 exits laser gain module6024. Laser beam segments 6000, 6006, 6010, 6012, and 6014 combine toform a main laser beam which is distinct from pump beams 6018.

The number of times that the main laser beam crosses gain slab 6002depends on the relative positioning of input laser beam segment 6022,TIR mirror 6004, and TIR mirror 6008. Main laser beam segments 6000,6006, 6010, 6012, and 6014 travel along paths that are substantiallyparallel to the Z direction. Pump beams 6018 travel along paths that aresubstantially parallel to the X direction.

FIG. 61 shows a method of using retroreflective mirrors for opticalamplification. In step 6100, a pump laser beam is injected into a lasergain slab. In step 6102, a main laser beam is injected into the lasergain slab. In step 61061, the main laser beam is reflected from a firstretroreflecting minor. In step 6106, the main laser beam is amplified bypassage through the laser gain slab. In step 6108, the main laser beamis reflected from a second retroreflecting mirror. In step 6110, themain laser beam is again amplified by passage through the laser gainslab. Steps 6104 through 6110 are repeated multiple times. In step 6112,the amplified main laser beam is output from the laser gain slab.Injecting is defined as focusing or otherwise introducing a beam oflight into the laser gain slab. Lenses may be used to focus the beaminto the laser gain slab.

Flat minors reflect light such that the angle of incidence equals theangle of reflection. In contrast, retroreflective mirrors reflect a beamof light back in the same direction as the incident direction.Retroreflective minors in two dimensions may be formed by placing twoflat mirrors at right angles. Retroreflective minors in three dimensionsmay be formed by placing three flat mirrors at right angles to form acorner-cube reflector. TIR prisms are a form of retroreflective minorsthat use total internal reflection to form completely reflective mirrorson certain faces of the prism. When the index of refraction of the prismmaterial is sufficiently high and the angle of the incident light on theface of the prism is sufficiently high, rays of light in the prismcannot exit the faces of the prism and are instead totally internallyreflected according to Snell's law. To form TIR prisms that operate atinternal incident angles of 45 degrees (such as TIR prisms 6004 and6008), the index of refraction must be greater than 1.41 at thewavelength of the main laser beam. Many optical glass and crystalmaterials satisfy this index of refraction criterion. TIR prisms 6004and 6008 are two-dimensional retroreflective minors. TIR prisms 6004 and6008 may alternately be replaced by other types of retroreflectiveminors such as high-reflection flat minors positioned at right angles.Another type of TIR prism is an immersed design where the light travelsin a high index material, but instead of being surrounded by air as inthe previous examples, the high index material is surrounded by a lowerindex material such as low index glass on the sides of the prismdesigned to have total internal reflection.

External laser 6022 in FIG. 60 provides light pulses or continuous wavelight to laser gain module 6024. External laser 6022 may be a Q-switchedND:YLF laser which produces light at 1047 nm or a Q-switched ND:YAPlaser which produces light at 1079.5 nm. The optical power may beapproximately 2 watts. The Q-switching frequency may be 125 kHz with arange of 20 kHz to 300 kHz, and the pulse width may be 25 nm with arange of 5 ns to 100 ns. The beam spatial profile may be Gaussian oranother beam shape.

The gain slab carries the main laser beam inside while the main laserbeam is reflecting back and forth between the two retroreflectivemirrors. The gain slab is composed of a medium which allows stimulatedemission. The gain slab may composed of the same material as thematerial used for the external laser. For example, if the external laseris of the ND:YLF type, the gain slab may also be composed of ND:YLF. Ifthe external laser is of the ND:YAP type, the gain slab may also becomposed of ND:YAP.

Pump lasers 6016 may be laser diode bars which may have collimationoptics to guide their light into gain slab 6002 and efficiently from again sheet in gain slab 6002. The collimation optics may besubstantially cylindrical lenses that focus the beam primarily in the Ydirection. Pumps lasers 6016 may be collimated in the Y direction sothat the gain sheet is well defined and has a quasi-Gaussiandistribution of gain in the Y direction while being very uniform in theX-Z plane if the gain-slab absorption coefficient is suitably chosen.The optical power of the pump lasers may be over 1 watt per laser gainmodule, and the wavelength may be 808 nm.

An advantage of laser gain module 6024 relative to laser gain module5824 is that laser gain module 6024 more uniformly fills the gain sheetvolume with the main laser beam. In laser gain module 5824, the mainlaser beam is squeezed into a small volume in the regions near thereflection from minors 5804 and 5808. In these regions, the main laserbeam overly depletes the population of excited atoms created by pumpbeams 5818. Other areas of the gain sheet are not filled by the mainbeam and therefore do not contribute to the optical amplification of themain beam. In laser module 6024, the main beam traverses the gain sheetvolume more uniformly and therefore has the chance to utilize the fullpopulation of excited atoms in a large fraction of the gain sheet. Inthe Z-direction, also called the longitudinal direction, the uniformityof the main laser beam power density is improved by usingretroreflective mirrors rather than flat minors. The components of lasergain module 6024 may be suitably designed to optimize the utilization ofthe excited atoms in the gain sheet by distributing the main laser beamuniformly throughout the gain sheet volume.

Another advantage of laser gain module 6024 relative to laser gainmodule 5824 is that laser gain module 6024 does not have parallel minorsor minors close to parallel. Parallel or close to parallel mirrors suchas mirrors 5804 and 5808 in laser gain module 5824 produce parasiticlaser activity because a resonance condition is set up where spontaneousemission in gain slab 5802 will result in laser emission that depletesthe excited atoms in gain slab 5802. Any parasitic laser activityremoves excited atoms from the population of excited atoms in gain slab5802 so those atoms are no longer available to optically amplify themain beam. To avoid parasitic laser activity, minors 5804 and 5808 mustbe slightly misaligned in both θ and φ angles. The misalignment isdifficult to control and depends to some extent on the wedge angle ofthe optical faces of gain slab 5802. Laser gain module 6024 replacesminors 5804 and 5808 with TIR prisms that have no parallel or close toparallel faces. In addition, antireflection coatings at the lasingwavelength may be used on the end faces of gain slab 6002, and the endfaces of the gain slab may be tilted to prevent additional parallelsurfaces that can cause parasitic laser activity. The end faces of gainslab 6002 are the two faces that are closest to TIR prisms 6004 and6008.

Multiple laser gain modules 6024 may be made interchangeable and easilyreplaceable by suitable alignment of gain slab 6002, TIR prisms 6004 and6008, and other components of laser gain module 6024. An externalalignment fixture may be used to align the components of laser gainmodule 6024 to produce a fixed and constant relationship between inputlaser beam segment 6000 and output laser beam segment 6014 for multiplelaser gain modules. This allows for easy replacement of defectivemodules and easy alignment of the overall optical system that uses lasergain modules 6024.

Multiple laser gain modules 5824 cannot control output beam angle andposition relative to the input accurately enough to chain laser gainmodules 5824 without using location adjustments and mirror tilts ofsuccessive modules to correct errors of preceding modules. In otherwords, laser gain modules 5824 are not interchangeable. Multiple lasergain modules 6024, on the other hand, may be easily combined in seriesto amplify the main beam more than can be achieved by using a singlelaser gain module 6024. The limit of multiple gain modules is determinedprimarily by surface damage issues, not bulk damage in the gain sheets.But when the main beam reaches a sufficiently high power, the main beammay be optically steered by thermally induced changes in the index ofrefraction of the gain sheets. Compensation for beam steerage may beperformed by additional optical elements dedicated to that task.

Thermal issues may affect the performance or alignment of the laser gainmodule if the laser gain module gets too hot or if the temperature ofthe module is not held sufficiently constant. Water cooling may be usedto cool the laser gain module. Stability may be improved by splittingthe water cooling into sections so that different parts of the lasergain module are cooled by different water cooling circuits.

The alignments of mirrors 5804 and 5808 in laser gain module 5824 arevery sensitive to minor tilts and small errors which may lead toinefficient operation of laser gain module 5824. The alignment of lasergain module 6024 is much easier to perform because there are no anglemisalignments in θ or φ required to avoid parasitic laser activitytherefore the prisms may be aligned in the module with only relativealignments between the prisms in the X direction required. Even thisalignment may be eliminated by templating if the parts of laser gainmodule 6024 are toleranced properly. Also, angle misalignments of TIRprisms 6004 and 6008 do not affect the angle of output laser beam 6014,only its location and beam separation from input laser beam 6000. Anadditional feature of laser gain module 6024 is that the number timesthe main laser beam passes through gain slab 6002 can be easily adjustedby changing the relative displacement of TIR prisms 6004 and 6008 in theX direction. Displacement of TIR prisms 6004 and 6008 in the X directionmay also be used to null out any error in alignment of the components oflaser gain module 6024.

In another aspect of the optical system shown in FIG. 1, an optical tap108 may be formed to monitor and control a high-intensity beam of light.By using two uncoated glass plates, laser damage is avoided and thealignment of the original beam is not affected. Specific embodiments aredescribed in the following paragraphs but are not meant to be limitingin any way.

Optical systems with high intensity light beams may experience drift orother instabilities. To enable feedback and control of the optical beamintensity, an optical tap may be utilized to monitor the high intensitybeam. An optical tap takes a small amount of light from a high-intensitylight beam and directs it to a detector where the small amount of lightis converted to an electrical signal that represents the intensity ofthe high-intensity light beam.

FIG. 62 shows an optical system with an optical tap. First lightprocessing element 6200 provides first beam segment 6202. First beamsegment 6202 is collimated by first lens system 6204 to form second beamsegment 6206. Second beam segment 6206 enters optical tap 6208. Opticaltap 6208 outputs electrical signal 6218 which is used by electricalcircuit 6220. Third beam segment 6210 exits optical tap 6208 and isfocused by second lens system 6212 to form fourth beam segment 6214.Fourth beam segment 6214 enters second light processing element 6216. Inthis example, first beam segment 6202 and fourth beam segment 6214 areun-collimated beams of light, whereas second beam segment 6206 and thirdbeam segment 6210 are collimated beams of light. Alternatively, allbeams may be collimated so that first lens system 6204 and second lenssystem 6212 are not necessary. First light processing element 6200 maybe a light source such as a laser, or it may be any optical componentthat provides a beam of light. Second light processing element 6216 maybe any optical component that utilizes the light from first lightprocessing element 6200. The final optical output of the optical systemmay be third beam segment 6210 in which case, there is no second lightprocessing element 6216. Electrical signal 6218 is related to theintensity of second beam segment 6206. Electrical signal 6218 may beproportional to the intensity of second beam segment 6206. Electricalcircuit 6220 may be a feedback circuit, control circuit for first lightprocessing element 6200, or any other electrical circuit that useselectrical signal 6218 as an input.

FIG. 63A shows the detailed workings of an optical tap with two plates.Such a tap may be utilized by the optical system of FIG. 62. First beamsegment 6300 partially reflects from first plate 6302 to form secondbeam segment 6314 and third beam segment 6304. Second beam segment 6314is absorbed by beam dump 6316. Third beam segment 6304 is inside firstplate 6302. Third beam segment 6304 exits first plate 6302 to formfourth beam segment 6306. Fourth beam segment 6306 partially reflectsfrom second plate 6310 to form fifth beam segment 6318 and sixth beamsegment 6308. Fifth beam segment 6318 is a small fraction of theintensity of first beam segment 6300. Fifth beam segment 6318illuminates detector 6320 which produces electrical signal 6330. Sixthbeam segment 6308 is inside second plate 6310. Sixth beam segment 6308exits second plate 6310 to form seventh beam segment 6312. First plate6302 and second plate 6310 may be composed of transparent solidmaterials such as glass or optical crystal. First beam segment 6300 is acollimated light beam. Second order reflections which produce beamsegments of lower intensity are not shown in FIG. 63A. Second plate 6310steers seventh beam segment 6312 so that it is co-linear with first beamsegment 6300. Co-linear means that two beams lie on the same line. Theangle of incidence of first beam 6300 on first plate 6302 is equal tothe angle of incidence of fourth beam 6306 on second plate 6310. Firstplate 6302, second plate 6310, beam dump 6316, and detector 6320 formoptical tap 6340. Alternatively, beam dump 6316 may not be included inthe optical tap, but its function of absorbing second beam segment 6314may instead be provided by the general enclosure (not shown) of opticaltap 6340. Alternatively, beam dump 6316 and detector 6320 may bereversed so that the monitored reflection is from the first plate ratherthan the second plate.

FIG. 63B shows the detailed workings of an optical tap with threeplates. Such a tap may be utilized by the optical system of FIG. 62.First beam segment 6342 partially reflects from first plate 6344 to formsecond beam segment 6350 and third beam segment 6346. Second beamsegment 6350 is absorbed by first beam dump 6352. Third beam segment6346 is inside first plate 6344. Third beam segment 6346 exits firstplate 6344 to form fourth beam segment 6348. Fourth beam segment 6348partially reflects from second plate 6354 to form fifth beam segment6360 and sixth beam segment 6356. Sixth beam segment 6356 is insidesecond plate 6354. Sixth beam segment 6356 exits second plate 6354 toform seventh beam segment 6358. Fifth beam segment 6360 partiallyreflects from third plate 6362 to form eighth beam segment 6370 andninth beam segment 6364. Ninth beam segment 6364 is inside third plate6362. Ninth beam segment 6364 exits third plate 6362 to form tenth beamsegment 6366. Tenth beam segment 6366 is absorbed by second beam dump6368. Eighth beam segment 6370 is a small fraction of the intensity offirst beam segment 6342. Eighth beam segment 6370 illuminates detector6372 which produces electrical signal 6374. First plate 6344, secondplate 6354, and third plate 6362 may be composed of transparent solidmaterials such as glass or optical crystal. First beam segment 6342 is acollimated light beam. Second order reflections which produce beamsegments of lower intensity are not shown in FIG. 63B. Second plate 6354steers seventh beam segment 6358 so that it is co-linear with first beamsegment 6342. The angle of incidence of first beam 6342 on first plate6344 is equal to the angle of incidence of fourth beam 6348 on secondplate 6354. First plate 6344, second plate 6354, third plate 6362, firstbeam dump 6352, second beam dump 6368, and detector 6372 form opticaltap 6380. Alternatively, first beam dump 6352 and second beam dump 6368may not be included in the optical tap, but the function of absorbingsecond beam segment 6350 and tenth beam segment 6366 may instead beprovided by the general enclosure (not shown) of optical tap 6380.Alternatively, first beam dump 6352 may be exchanged with third plate6362, second beam dump 6368, and detector 6372 so that the monitoredreflection is from the first plate and the third plate rather than thesecond plate and the third plate. The optical tap with three platesshown in FIG. 63B can be used to monitor higher power beams withoutsaturation of the detector compared to the optical tap with two platesshown in FIG. 63A.

FIG. 64 shows an optical plate with an incident ray of light. The beamsegments in FIGS. 63A and 63B consist of multiple rays of light whicheach act as shown in FIG. 64 when incident on first plate 6302 andsecond plate 6310. First ray segment 6402 enters plate 6400 and ispartially refracted to form second ray segment 6412 and partiallyreflected to form third ray segment 6408. First angle 6406 is the anglebetween first ray segment 6402 and plate perpendicular 6404. First angle6406 is the incidence angle of first ray segment 6402. Second angle 6410is the angle between third ray segment 6408 and plate perpendicular6404. Second angle 6410 is the reflection angle of second ray segment6408. The magnitude of first angle 6406 is equal to the magnitude ofsecond angle 6410. Second ray segment 6412 passes through plate 6400until it reaches the back surface of plate 6400 where it exits plate6400 and is partially refracted to form fourth ray segment 6414 andpartially reflected to form fifth ray segment 6420. Fourth ray segment6414 propagates in the same direction as first ray segment 6402, but isoffset by distance 6418 from path 6416 that first ray segment 6402 wouldhave taken in the absence of plate 6400. Fifth ray segment 6420 passesthrough the plate 6400 until it reaches the front surface of plate 6400where it exits plate 6400 and is partially refracted to form sixth raysegment 6422 and partially reflected to form seventh ray segment 6424.Seventh ray segment 6424 continues reflecting back and forth insideplate 6400 growing weaker each time. Third ray segment 6408, sixth raysegment 6422, and similar ray segments (not shown) past seventh raysegment 6424 are reflected from plate 6400 and in combination make anoverall reflected beam from plate 6400. Fourth ray segment 6414 andsimilar ray segments (not shown) past seventh ray segment 6424 aretransmitted through plate 6400 and in combination make an overalltransmitted beam through plate 6400.

FIG. 65 shows a graph of reflection from a plate such as first plate6302, second plate 6310, first plate 6344, second plate 6354, thirdplate 6362, and plate 6400 in FIGS. 63A, 63B, and 64. The x-axisrepresents the angle of incidence in degrees and the y-axis representsthe intensity of the reflection in percent of the intensity of theincident beam. First curve 6500 shows the reflection of s-polarizedlight and second curve 6502 shows the reflection of p-polarized light.S-polarized light has a large reflection at all angles of incidence, butthe reflection of p-polarized light is close to zero for angles close to56 or 57 degrees. The angle of zero reflection for p-polarized light isknown as Brewster's angle. In the example of FIG. 65, the index ofrefraction of the plate is 1.52. If the index of refraction isdifferent, Brewster's angle will also be different.

FIG. 66 shows an expanded graph of FIG. 65 centered near Brewster'sangle. For many applications, a tap reflection of between 1×10⁻⁴ of1×10⁻⁷ is an appropriate amount of power for the detector to acceptwithout saturation on the high end or too much noise on the low end.Curve 6600 shows the reflection of p-polarized light. An effective rangeof angles for the optical tap may lie between 56.3 degrees and 57.0degrees. In order to produce a tap reflection of 1×10⁻⁵, the angle ofincidence must be close to 56.45 degrees or 56.87 degrees. In order toproduce a tap reflection of 1×10⁻⁶, the angle of incidence must be closeto 56.59 degrees or 56.72 degrees. If the high intensity beam beingmonitored is 100 watts these reflections make 1 milliwatt and 100microwatts respectively, which is within the optimum range ofconventional semiconductor detectors such as PIN (P-doped, Intrinsic,N-doped) photodiodes. The small amount of light removed from the highintensity beam does not significantly affect its use in mostapplications.

FIG. 67 shows a light generation system that uses three optical taps tocontrol the output color. The three optical taps monitor threehigh-intensity beams of light and electronic control is used to keep theoutput color at a desired color point. First light processing element6700 provides first beam segment 6702. First beam segment 6702 iscollimated by first lens system 6704 to form second beam segment 6706.Second beam segment 6706 enters optical tap 6708. Optical tap 6708outputs first electrical signal 6718 which is used by electrical circuit6760. Third beam segment 6710 exits optical tap 6708 and is focused bysecond lens system 6712 to form fourth beam segment 6714. Fourth beamsegment 6714 enters second light processing element 6716. Third lightprocessing element 6720 provides fifth beam segment 6722. Fifth beamsegment 6722 is collimated by third lens system 6724 to form sixth beamsegment 6726. Sixth beam segment 6726 enters optical tap 6728. Opticaltap 6728 outputs second electrical signal 6738 which is used byelectrical circuit 6760. Seventh beam segment 6730 exits optical tap6728 and is focused by fourth lens system 6732 to form eighth beamsegment 6734. Eighth beam segment 6734 enters fourth light processingelement 6736. Fifth light processing element 6740 provides ninth beamsegment 6742. Ninth beam segment 6742 is collimated by fifth lens system6744 to form tenth beam segment 6746. Tenth beam segment 6746 entersoptical tap 6748. Optical tap 6748 outputs third electrical signal 6758which is used by electrical circuit 6760. Eleventh beam segment 6750exits optical tap 6748 and is focused by sixth lens system 6752 to formtwelfth beam segment 6754. Twelfth beam segment 6754 enters sixth lightprocessing element 6756. First light processing element 6700, first lenssystem 6704, optical tap 6708, second lens system 6712, second lightprocessing element 6716, third light processing element 6720, third lenssystem 6724, optical tap 6728, fourth lens system 6732, fourth lightprocessing element 6736, fifth light processing element 6740, fifth lenssystem 6744, optical tap 6748, sixth lens system 6752, sixth lightprocessing element 6756, and electrical circuit 6760 form lightgeneration system 6770.

In the example of FIG. 67, first beam segment 6702, fourth beam segment6714, fifth beam segment 6722, eighth beam segment 6734, ninth beamsegment 6742, and twelfth beam segment 6754 are un-collimated beams oflight, whereas second beam segment 6706, third beam segment 6710, sixthbeam segment 6726, seventh beam segment 6730, tenth beam segment 6746,and eleventh beam segment 6750 are collimated beams of light.Alternatively, all beams may be collimated so that first lens system6704, second lens system 6712, third lens system 6724, fourth lenssystem 6732, fifth lens system 6744, and sixth lens system 6752 are notnecessary. First light processing element 6700, third light processingelement 6720, and fifth light processing element 6740 may be lightsources such as lasers, or may be any optical components that provide abeam of light. Second light processing element 6716, fourth lightprocessing element 6736, and sixth light processing element 6756 may beany optical components that utilize the light from first lightprocessing element 6700, third light processing element 6720, and fifthlight processing element 6740 respectively. The final optical output ofthe optical system may be a combination of third beam segment 6710,seventh beam segment 6730, and eleventh beam segment 6750 in which case,there is no second lens system 6712, fourth lens system 6732, sixth lenssystem 6752, second light processing element 6716, fourth lightprocessing element 6736, or sixth light processing element 6756. Firstelectrical signal 6718, second electrical signal 6738, and thirdelectrical signal 6758 are related to the intensity of second beamsegment 6706, sixth beam segment 6726, and tenth beam segment 6746respectively. First electrical signal 6718 may be proportional to theintensity of second beam segment 6706, second electrical signal 6738 maybe proportional to the intensity of sixth beam segment 6726, and thirdelectrical signal 6758 may be proportional to the intensity of tenthbeam segment 6746. Electrical circuit 6760 controls first lightprocessing element 6700, third light processing element 6720, and fifthlight processing element 6740 or other elements of the optical system(not shown) in order to balance the output color of light generationsystem 6770.

FIG. 68 shows a flowchart of a method of optical tapping. In step 6800,a first plate is illuminated with light. In step 6802, a small fractionof the light is reflected. In step 6804, the small fraction of light iscaptured in a detector. In step 6806, the remaining light is passed to asecond plate. In step 6808, the remaining beam is shifted to beco-linear with the first beam.

Conventional beamsplitters may consist of transparent plates of glasswith thin metal coatings or interference coatings that are operated atan incident angle of 45 degrees. For beam intensities greater thanapproximately 10 watts, it may not be practical to use a conventionalbeamsplitter to extract a portion of the beam for monitoring. Especiallywith pulsed laser systems such as Q-switched systems, the laser damagethreshold of conventional beamsplitters may be exceeded and damage tothe beamsplitter may result. In this case, the optical tap may beconstructed from an uncoated glass plate so that the laser damagethreshold of the plate is higher than the peak power of thehigh-intensity laser beam. The plates may be made of any material thatis transparent to the light beam being monitored. Optical glasses suchas BK7 may be used for their ruggedness and high transparency.

The amount of light reflected to the detector may be tuned by adjustingthe angle of the plates of the tap. This is equivalent to adjusting theincident angle of the incident beam. The angle of the second plate ofthe tap may also be adjusted to keep the beam co-linear by making theangle of the second plate equal to the angle of the first plate. Thetolerance of the angle is important to obtain the desired reflectionintensity. In the example of FIG. 66, if the desired reflection from theplate is 1×10⁻⁶, the nominal angle of incidence would be 56.594 degrees,but to maintain 1×10⁻⁶ plus or minus 10%, the angle would be 56.590degrees to 56.597 degrees. This is a range of 0.007 degrees. If thedesired reflection is 1×10⁻⁵, the nominal angle of incidence would be56.451 degrees, but to maintain 1×10⁻⁵ plus or minus 10%, the anglewould be 56.440 degrees to 56.462 degrees. This is a range of 0.022degrees. The beam being monitored should be collimated to within theangular range that gives the desired reflection. For example, if areflection of 1×10⁻⁶ is desired, the beam being monitored should becollimated to within approximately 0.007 degrees for proper operation ofthe optical tap at the nominal angle of 56.594 degrees.

In order for the optical tap to reflect a suitably small amount oflight, the high intensity beam being monitored must be close to 100%p-polarized. Even a small percentage of s-polarized light will cause alarge reflection because s-polarized light has a reflection ofapproximately 27% near Brewster's angle. S-polarized light should beless than approximately 1×10⁻⁶ of the high intensity beam if areflection on the order of 1×10⁻⁶ is desired at the nominal angle of56.594 degrees.

Applications of optical taps include any optical system wherehigh-intensity light beams must be monitored. Examples include laserprojectors, laser light shows, laser cutting, laser engraving, laserbeam processing of materials, and laser medical devices. Because theoptical tap does not significantly affect the high-intensity beam oflight in either beam path or intensity, the tap can be inserted orremoved into a high-intensity beam without degrading the operation ofthe light system being monitored. This makes a modular system that canbe easily adapted to existing optical designs without affecting thealignment of the optical system.

In another aspect of the optical system shown in FIG. 1, coupler 110 maybe used to reduce speckle. Three colors of light may be transferred fromlaser light sources and combined in a form which is easy to retrofit toprojectors that are designed for conventional light sources such as arclamps. Specific embodiments are described in the following paragraphsbut are not meant to be limiting in any way.

Digital projectors form projected digital images by modulating beams oflight to form pixels. The modulation is performed by light valves thatmay be reflective or transmissive. For large-venue applications, thelight sources must have high output power and high optical brightness,such as the conventionally used HID lamps. Laser light sources may alsobe used but are subject to visible speckle because of the long coherencelength of laser light as compared to HID lamps.

Speckle refers to a random pattern of small bright and dark spots thatis visible when laser light is reflected from a diffusing surface suchas a projection screen. The speckle pattern moves with the head of theviewer. The size of the spots appears to be at the limit of visibleresolution, and the spots may appear rainbow colored or may appear tohave other colors depending on the color of the projected light. Speckleis usually considered an undesirable side effect of laser illumination.

Speckle may be reduced by a number of methods including time diversity(also called phase diversity), path length diversity (also called anglediversity), and wavelength diversity. These techniques average thebright spot and dark spots of speckle over time, space, or wavelength inorder to reduce the amplitude of the brightness variations. A devicewhich reduces speckle is called a “despeckler”. A moving element of theoptical system such as a rotating diffuser will act as a despeckler aslong as the frequency of motion is high enough so that the movement isnot visible to the human eye. This is generally a movement frequencyhigher than approximately 100 Hz.

FIG. 69 shows a method of coupling a laser light source to a digitalimage projector. In step 6900, a laser light source generates a laserlight beam which illuminates an input lens system. In step 6902, theinput lens system collects and focuses the light beam so that itilluminates an optical fiber. In step 6904, the optical fiber passes thelight into the proper location to illuminate an output lens system. Instep 6906, the output lens system collects and focuses the light beam sothat it illuminates a despeckler. In step 6908, the despeckler reducesthe speckle of the light beam. In step 6910, the light beam passes intoan integrating rod which improves the spatial uniformity of the lightbeam. In step 6912, a digital image projector uses the light beam toform a projected digital image. The word “illuminate” is used to meanthat light is directly received by and passes into the object beingilluminated.

For digital image projectors based on DMD light valves, an integratingrod is typically incorporated as part of the standard projector design.For digital image projectors based on LCOS or LCD light valves, there istypically no integrating rod, but the optical coupler may require anadditional lens system to collimate the light beam before illuminatingthe projector.

FIG. 70 shows a side view of an optical coupler with a diffuser-baseddespeckler which is center driven. Laser light source 7002 generates alight beam which illuminates input lens system 7004. Input lens system7004 collects and focuses the light beam so that it illuminates core7008 of optical fiber 7006. Optical fiber 7006 passes the light beaminto the proper location to illuminate output lens system 7010. Outputlens system 7010 collects and focuses the light beam so that itilluminates diffuser 7012. Diffuser 7012 acts as a despeckler to reducethe speckle of the light beam and passes the light beam to illuminateintegrating rod 7018. Integrating rod 7018 improves the spatialuniformity of the light beam and passes the light beam so that isilluminates digital image projector 7020. Mechanical vibrator 7022vibrates optical fiber 7006 with core 7008 to provide additionaldespeckling. Diffuser 7012 is mounted on spindle 7014 which is rotatedby motor 7016 in a center-driven configuration. Input lens system 7004,optical fiber 7006 with core 7008, output lens system 7010, diffuser7012, vibrator 7022, spindle 7014, and motor 7016 form optical coupler7000. For clarity, mechanical support structures to hold the componentsand optical isolation structures are not shown.

FIG. 71 shows a side view of an optical coupler similar to the opticalcoupler in FIG. 70 except that diffuser 7112 is edge driven by spindle7114 and motor 7116. The edge-driven configuration may result in anoptical coupler of smaller size than the center driven configuration ofFIG. 70. Input lens system 7004, optical fiber 7006 with core 7008,output lens system 7010, diffuser 7112, vibrator 7022, spindle 7114, andmotor 7116 form optical coupler 7100.

FIG. 72 shows a side view of an optical coupler similar to the opticalcoupler in FIG. 70 except that a collimation lens system 7222 is addedso that the output of optical coupler 7200 forms collimated light beam7224 that illuminates projector 7220. No integrating rod is necessary inthis configuration, but projector 7220 may include light homogenizationcomponents such as a fly's eye lens. Input lens system 7004, opticalfiber 7006 with core 7008, output lens system 7010, diffuser 7012,vibrator 7022, spindle 7014, motor 7016, and collimation lens 7222 formoptical coupler 7200.

FIG. 73 shows a perspective view of the end of optical fiber bundle7300. Optical fiber bundle 7300 has two ends, only one of which is shownin FIG. 73. One central optical fiber 7306 and six surrounding opticalfibers 7302 are shown in optical fiber bundle 7300. Each surroundingoptical fiber 7302 has a surrounding core 7304. Central optical fiber7306 has central core 7308. Each surrounding core 7304 is capable ofcarrying light down the length of the corresponding surrounding opticalfiber 7302. Central core 7308 is capable of carrying light down thelength of central optical fiber 7306. Six surrounding optical fibers7302 fit well around central optical fiber 7306 in optical fiber bundle7300. Surrounding optical fibers 7302 and central optical fiber 7306 mayhave diameters in the range of 730 microns to 1000 microns and may bearranged within optical bundle 7300 in any geometric pattern.Surrounding cores 7304 have diameters larger than 50 microns, butsmaller than the corresponding surrounding optical fibers 7302. Centralcore 7308 has a diameter larger than 50 microns, but smaller than thecorresponding central optical fiber 7306.

FIG. 74 shows a side view of an optical coupler similar to the opticalcoupler in FIG. 70 except that an optical fiber bundle is used tocombine two laser light sources. First laser light source 7430 generatesfirst input light beam 7450 which illuminates first input lens system7434. First input lens system 7434 collects and focuses first inputlight beam 7450 to form first focused light beam 7454. First focusedlight beam 7454 illuminates first core 7444 of first optical fiber 7440.First optical fiber 7440 passes the light into the proper location toilluminate output lens system 7410 with first intermediate light beam7460. Second laser light source 7432 generates second input light beam7452 which illuminates second input lens system 7436. Second input lenssystem 7436 collects and focuses second input light beam 7452 to formsecond focused light beam 7456. Second focused light beam 7456illuminates second core 7446 of second optical fiber 7442. Secondoptical fiber 7442 passes the light into the proper location toilluminate output lens system 7410 with second intermediate light beam7462. Third laser light source 7470 generates third input light beam7472 which illuminates third input lens system 7476. Third input lenssystem 7476 collects and focuses third input light beam 7472 to formthird focused light beam 7476. Third focused light beam 7476 illuminatesthird core 7486 of third optical fiber 7484. Third optical fiber 7484passes the light into the proper location to illuminate output lenssystem 7410 with third intermediate light beam 7488.

Output lens system 7410 collects and focuses first intermediate lightbeam 7460, second intermediate light beam 7462, and third intermediatelight beam 7488 to form first output light beam 7464, second outputlight beam 7466, and third output light beam 7490. First output lightbeam 7464, second output light beam 7466, and third output light beam7490 illuminate diffuser 7012. Diffuser 7012 acts as a despeckler toreduce the speckle of first output light beam 7464, second output lightbeam 7466, and third output light beam 7490 and forms final light beam7468 which is a combination of first output light beam 7464, secondoutput light beam 7464, and third output light beam 7490. Final lightbeam 7468 illuminates integrating rod 7018. First input lens system7434, second input lens system 7436, third input lens 7474, firstoptical fiber 7440 with first core 7444, second optical fiber 7442 withsecond core 7446, third optical fiber 7484 with third core 7486, outputlens system 7410, diffuser 7012, vibrator 7022, spindle 7014, and motor7016 form optical coupler 7400. First optical fiber 7440 with first core7444, second optical fiber 7442 with second core 7446, and third opticalfiber 7484 with third core 7486 form optical fiber bundle 7482. Theoffset between first intermediate light beam 7460, second intermediatelight beam 7462, and third intermediate light beam 7488 and the offsetbetween first output light beam 7464, second output light beam 7464, andthird output light beam 7490 are exaggerated to show the individuallight beams more clearly. In the example of FIG. 74, three laser lightsources are shown, but two, four, or more laser light sources may alsobe combined in the same manner as shown in FIG. 74.

FIG. 75 shows a side view of an optical coupler similar to the opticalcoupler in FIG. 74 except that a dichroic beam combiner is used tocombine the two laser light sources instead of an optical fiber bundle.First laser light source 7532 generates first input light beam 7574which passes through first dichroic filter 7554 and second dichroicfilter 7534 to illuminate input lens system 7536. Second laser lightsource 7572 generates second input light beam 7552 which reflects fromfirst dichroic filter 7554 and passes through second dichroic filter7534 to illuminate input lens system 7536. Third laser light source 7530generates third input light beam 7570 which reflects from seconddichroic filter 7534 to illuminate input lens system 7536. Input lenssystem 7536 collects and focuses first input light beam 7554, secondinput light beam 7552, and third input light beam 7570 so that firstinput light beam 7554, second input light beam 7552, and third inputlight beam 7570 combine to form focused beam 7556 which illuminates core7544 of optical fiber 7540. Optical fiber 7540 passes the light into theproper location to illuminate output lens system 7510 with intermediatelight beam 7560. Output lens system 7510 collects and focusesintermediate light beam 7560 to form output light beam 7564. Outputlight beam 7564 illuminates diffuser 7012. Diffuser 7012 acts as adespeckler to reduce the speckle of output light beam 7564 and formsfinal light beam 7568. Final light beam 7568 illuminates integrating rod7018. First dichroic filter 7554, second dichroic filter 7534, inputlens system 7536, optical fiber 7540 with core 7544, output lens system7510, diffuser 7012, vibrator 7022, spindle 7014, and motor 7016 formoptical coupler 7500. First dichroic filter 7554 and second dichroicfilter 7534 form dichroic beam combiner 7580.

Laser light sources, such as those shown in FIGS. 70, 71, 72, 74, and75, may be semiconductor lasers, gas discharge lasers, diode-pumpedsolid state lasers, optical parametric oscillators, or other any othertype of light source which produces coherent light. The coherence lengthmay be on the order of 1 mm. Each laser light source may emit 30 W to1000 W of optical output. Q-switched lasers may be used to achieve highpower densities so that nonlinear optical processes may convert thewavelengths to all of the colors required by the digital imageprojector.

Diffusers, such as those shown in FIGS. 70, 71, 72, 74, and 75, may beformed from ground and/or etched glass, holographic methods, or bulkdiffusing materials. A high damage threshold is helpful to handle thelarge optical flux at the focal point of the beam on the diffuser. Arotating diffuser helps spread the heat load over a larger area so thatthe diffuser is self-cooling. With proper design of the diffuser, 7300watts or more of optical power may be transmitted through the diffuserwithout overheating. In a well-designed optical coupler, as the opticalpower of the laser light sources is increased, at some point the opticalabsorption of the light valves or other components in the projector willbecome the limiting factor for overall system output. Although thediffuser slightly increases the spread of the light beam passing throughit, by keeping the diffuser close to the integrating rod, most of thelight that exits the diffuser will be captured by the acceptance cone ofthe integrating rod.

Diffusers reduce speckle even if not in motion due to path lengthdiversity, but they reduce speckle more when rotated or otherwise movedbecause time diversity adds further averaging. The optimal rotationfrequency depends on the size of the diffuser features, but greater than50 rpm is typically sufficient to optimize the speckle reduction.

Optical fibers that carry a high flux of light should optimally have lowabsorption for the wavelength region in the light beam. SiO₂ fibers maybe used for this purpose. Fibers with a high concentration of hydroxylradicals have reduced absorption in the blue region of the spectrum.Optical fiber bundles may be used such that the exit beams from thefibers are displaced by only a fraction of a millimeter. Keeping theexit beams close together is advantageous because the beams combine wellinto one beam that still has high throughput through the projectoroptical system. The laser light beam illuminating each fiber may havemore than 10 watts of optical power when used by digital cinemaprojectors for large venues. By using seven optical fibers and laserlight sources of 150 watts each, the total optical output from thecoupler may be on the order of 800 watts after accounting for losses inthe optical coupler. The fiber may be vibrated in order to performdespeckling. The vibration frequency may be greater than 100 Hz, and thevibration apparatus may be a piezo-electric actuator. The length of thefiber may be in the range of 1 to 50 meters to achieve suitabledespeckling.

Lens systems may be a single lens, or a combination of multiple lenselements. Output lens system 7010 in FIG. 70 may be designed as aone-to-one imaging system so that the beam diameter at the output ofoptical fiber 7006 approximately equals the beam diameter at the inputsurface of diffuser 7012. The design of input lens system 7004 dependson the properties of the light beam exiting laser light source 7002.Input lens system 7004 needs to focus the beam of light from laser lightsource 7002 so that the light enters core 7008 of optical fiber 7006.Ray tracing software such as Zemax (Zemax Development Corporation,Bellevue, Wash.) may be used to design lens systems that effectivelycollect and focus light beams into the required locations.

Optical fiber bundle 7482 in FIG. 74 is one example of a beam combiner.Beam combiners may operate on various principles depending on theoptical characteristics of the light beams being combined. Theseprinciples may include geometric combination, polarization combination,or wavelength combination. Fiber bundle 7482 operates on the principleof geometric combination. If the light beams are of differentpolarization states, the beam combiner may include a reflectivepolarizer. In this case, one polarization of light is transmitted andone is reflected. Another example is shown in FIG. 75 where the lightbeams have different colors. In this example, the beam combiner mayconsist of two dichroic filters that together form a dichroic beamcombiner. In each filter, one region of colors is transmitted andanother region of colors reflected. By using two filters, three colorsmay combined into one beam. Multiple beam combiners may be added inseries or parallel to combine any number of light beams.

If feedback is necessary, an optical sensor may be incorporated into theoptical coupler to sense the intensity of light passing through theoptical coupler. The optical sensor may be a photodiode sensitive at thewavelengths of light used in the optical coupler. The sensor may belocated at any convenient location. One such location is between outputlens system 7010 and diffuser 7012 in FIG. 70.

An integrating rod may be formed from a rectangular piece of glass thatconfines the light inside using total internal refraction. Assuming therod has sufficient length, light that enters with any spatial profile,but at angles below the acceptance angle of the integrating rod, willreflect inside the rod and become uniformly spread so that it exits therod with a top-hat spatial distribution. The entry face of theintegrating rod may be approximately 1 cm on the longer side forlarge-venue cinema projectors. The aspect ratio of the entry-face longerside to the entry-face shorter side may be the same aspect ratio as thelight valve in order to homogenize the light over the aspect ratio ofthe light valve.

The optical couplers shown in FIGS. 70, 71, 72, and 74 which incorporateboth vibrating fibers and rotating diffusers are capable of reducing thespeckle to less than 4%. In large-venue applications, speckle of 4% orless may be considered acceptable and speckle of 1% or less is typicallyinvisible to the naked eye.

The maximum power that can be coupled into one fiber is generallylimited by the damage threshold of the input face of the fiber. Theoptical coupler described herein uses multiple fibers and/or beamcombiners to enable higher optical power to be efficiently and reliablycoupled into a digital projector. The multiple fibers or beam combinersmay also be used to combine different colors of light such as red,green, and blue into a digital projector.

One example of the use of an optical coupler is the after-marketreplacement of high-intensity-discharge lamps with laser light sourcesfor off-the-shelf cinema projectors. These high-intensity-dischargelamps are usually xenon lamps with short life spans. The optical couplermay be designed so that the laser light source is a drop-in retrofit toincrease the optical output, light source lifetime, and power efficiencywhen compared to the original xenon bulb. The replacement may be assimple as removing the existing xenon bulb, and bolting the opticalcoupler into the area where the xenon lamp was mounted. The opticalcoupler conveniently combines the functions of coupling, color mixing,and despeckling into one subassembly. The alignment of the opticalcoupler is not sensitive to position and angle. As long as the outputbeam of the optical coupler enters the acceptance aperture andacceptance cone of the projector, the light from the optical couplerwill be effectively coupled into the projector. A connector may be usedto connect the optical fiber to the supporting structure than holds thefiber. The connector allows quick connection and disconnection of theoptical coupler. The connector may be an SMA connector. Whether there isone optical fiber or a bundle of optical fibers, the same SMA connectormay be used to provide a modular capability to increase power bychanging optical couplers. No optical alignment is needed when attachingor changing the fibers with SMA connectors. Other types of lamps, suchas mercury vapor lamps or tungsten lamps, may also be replaced by laserlight sources in a similar manner.

The maximum repetition rate of a single Q-switched laser is limitedbecause higher repetition rate generally means lower peak power in eachpulse. High peak power is desirable for the nonlinear effects used incolor conversion, but may not be achievable if the repetition rate istoo high. On the other hand, a high repetition rate is desirable toavoid creating beat frequencies with DMD light valves or other periodicevents in digital image projectors. The optical coupler described hereinenables multiple laser light sources to be combined so that therepetition rate is high while maintaining high peak power. For example,two Q-switched lasers running at repetition rates of 100 kHz each can becombined without sacrificing peak power to have an effective repetitionrate of 200 kHz.

In another aspect of the optical system shown in FIG. 1, flat-sidedfiber 112 may be used to further reduce speckle. The flat-sided fibermay be used as one part of optical couplers such as those shown in FIGS.70, 71, 72, 74, and 75. Specific embodiments are described in thefollowing paragraphs but are not meant to be limiting in any way.

Optical fibers with rectangular cores are conventionally used forapplications such as making rectangular power distributions. Thispurpose is shown in FIGS. 1 through 4 of Japanese patent publication No.2003121664, published Apr. 23, 2003.

FIG. 76 shows the optical layout of an optical system with a laser lightsource, an optical fiber, and a digital image projector. Laser lightsource 7600 outputs laser light which is focused by input lens system7602 so that it illuminates core 7606 of optical fiber 7604. The outputof optical fiber 7604 is focused by output lens system 7608 so that itilluminates digital image projector 7610. Projector lens 7612 formsimage 7616 on screen 7614. The digital image projector may include amixing rod in which case output lens system 7608 couples the output ofoptical fiber 7604 into the mixing rod. Input lens system 7602 andoutput lens system 7608 may consist of any combination of opticalelements such as lenses and minors that are able to transfer light intoand out of the fiber. Laser light source 7600 may be a single laser ormay include multiple lasers of different colors or with various opticalproperties. Optical fiber 7604 allows flexibility in the location oflaser light source 7600 and digital image projector 7610 while allowingeasy alignment of these two parts. Laser light source 7600 and digitalimage projector 7610 may be located far from each other, limited only bythe length of optical fiber 7604. Alternately, optical fiber 7604 may becoiled such laser light source 7600, digital image projector 7610, andoptical fiber 7604 are located in close proximity.

FIG. 77 shows a cross sectional view of a flat-sided fiber. Core 7700has flat side 7710, height 7704 and width 7706. Glass cladding 7702surrounds core 7700 and forms air cladding region 7708 adjacent to flatside 7710. Other than air cladding region 7708, the rest of core 7700has a conventional glass cladding in contact with core 7700. Surroundingand in contact with cladding 7702 is a protective coating 7712 which istypically an acrylate or polyimide though other coatings may be used.The index of refraction of core 7700 is higher than the index ofrefraction of cladding 7702 or air cladding region 7708. Light travelsprimarily in core 7700 and is guided by TIR from the interface betweencore 7700 and cladding 7702 or air cladding region 7708. Cladding 7702fulfills the function of keeping the light in the optical fiber core.Coating 7712 protects the fiber from contamination, abrasion, or otherdegradation from the environment outside core 7700. Alternately, glasscladding 7702 may be in contact with core 7700 all the way around core7700 without an air cladding region. The term “flat-sided fiber” is usedfor optical fibers that have a flat-sided core even though the outsideof the fiber may not have a flat side.

FIG. 78 shows a cross sectional view of a rectangular optical fiber withglass cladding. A rectangular core fiber is a specific case of aflat-sided optical fiber where there are four flat sides and the foursides are perpendicular. Core 7800 has a rectangular cross section withheight 7804 and width 7806. Glass cladding 7802 surrounds core 7800 andis covered by coating 7812. The operation of the rectangular opticalfiber is similar to the one-sided optical fiber shown in FIG. 77 exceptthat there are 4 flat sides instead of one flat side and there is no aircladding region. In this example, the ratio of width 7806 to height 7804is 2:1. Other ratios may be used to match the aspect ratio of the lightvalves in the digital image projector. For example, 4:3 and 16:9 may beused if the light valves have one of those aspect ratios.

FIG. 79 shows a cross sectional view of a rectangular optical fiber withair cladding. Core 7900 has a rectangular cross section with a height7904 and a width 7906. Glass cladding 7902 forms air cladding regions7908 because glass cladding 7902 surrounds core 7900 but only touchescore 7900 at the corners of core 7900. Coating 7912 protects the fiber.In this example, the ratio of width 7906 to height 7904 is 2:1. Theoperation of the rectangular optical fiber is similar to the rectangularoptical fiber shown in FIG. 3 except that there are air cladding regionssurrounding all four sides of the rectangular core.

FIG. 80 shows a method of illuminating a digital projector with aflat-sided fiber. In step 8000, coherent light is generated from a laserlight source. In step 8002, a flat-sided optical fiber is illuminatedwith the coherent light from the laser light source. In step 8004, adigital image projector is illuminated with the output from theflat-sided fiber. A flat-sided optical fiber refers to a fiber with aflat-sided core. The core may have one or more flat sides. If there arefour flat sides perpendicular to each other, the core will have arectangular cross section.

The fiber cores shown in FIGS. 77 through 79 are shown with sharp edges,but due to the fiber draw process as well as typical sleeving processesused in conventional fiber manufacture, the sharp edges will be softenedslightly at the corners due to surface tension self-rounding effects.The cladding is shown fully collapsed around the core in FIG. 3, butthis is not required as alternatively shown in FIGS. 77 and 79. The cladneed only ensure that the light stays in the core.

One or more flat sides on the core give a number of benefits forilluminating a digital image projector with a laser light source. Oneadvantage of this shape core is that the source is made highly uniformafter propagating a short distance. The lack of circular symmetry causesa randomization of the input light distribution effectively making atop-hat intensity pattern the same size as the core. In the case of arectangular fiber, from the physical optics point of view the spatialmodes of the rectangular fiber do not individually overlap an inputcircularly symmetric beam well which means that a large number of guidedmodes are excited which causes a broad energy distribution. Since thesemodes dephase as they propagate with different modal indices, theoriginal light distribution is lost and the light output more uniformlycovers the fiber field. Since the rectangular fiber core may be matchedin aspect ratio to the modulators used in a digital projector, themodulators may be uniformly filled.

Another benefit of the flat-sided core is speckle reduction in lasersources whose spectral bandwidth is at least a few tenths of ananometer. If the spectral bandwidth is at least 1 nm wide, there may bevery significant speckle reduction. Because a large number of modes areexcited in a flat-sided fiber and each travels with its own phasevelocity, provided the fiber is long enough, the modes will eventuallybe delayed by an amount greater than the source coherence length and thespeckle will be reduced roughly by the square root of the number of themodes excited. This is a diminishing return effect and for a givensource bandwidth, the fiber length must be balanced against specklereduction, attenuation, and cost. The fiber length may be in the rangeof 1 to 500 meters. For the best balance of cost and speckle reduction,the fiber length may be in the range of 5 to 50 meters.

As a specific example of speckle reduction, for a laser light sourcewith a central wavelength of 523 nm and a spectral bandwidth of 1 nm, a100 micrometer by 200 micrometer rectangular-core fiber with a numericalaperture of 0.22 would be capable of supporting approximately 5000modes. This implies that the best speckle reduction possible would beabout 18 dB if every mode was incoherent and equally excited. Thedistance required to achieve this in a straight fiber would be verylarge but one could achieve approximately 13 dB reduction after only 20meters.

In the case of fiber lengths that do not fully exceed the coherencelength of modes with adjacent propagation constants, inter-modalinterference will occur leading to a speckle-like spatial intensitypattern at the fiber output. Since the fiber acts like a longinterferometer in this case, time averaging by perturbing the fibereliminates this residual noise pattern. This may be accomplished by avariety of methods such as using a vibrating element to vibrate asection of the fiber thus temporally changing the relative mode phasedelays at a rate sufficiently higher than the observer's detection rateto filter the residual noise. The vibrating element may be apiezoelectric transducer.

An advantage of a rectangular core is that it is capable of preservingthe polarization of input light. This will be possible if the inputpolarization is aligned parallel to one of the core edges. This can beunderstood from the point of view of a ray propagating down the fiber.Since each total internal reflection is essentially a specularreflection from a flat surface, the polarization of the ray isunchanged.

The material of the fiber may be glass, plastic, or any materialtransparent at the wavelengths of operation. Glass is preferred for highpower use due to its low absorption and high damage threshold. Silicaglass with a high concentration of hydroxyl or deuteroxyl radicals hasreduced absorption in the blue region of the spectrum. The maximum powerthat may be inserted into the fiber will be limited by laser damagethreshold as well as possible nonlinear effects. Laser damage thresholdof the cleaved or polished faces can be avoided if the power density iskept below a specific value such as 200 megawatts per cm². Examples ofnonlinear effects which could limit power carrying capability arestimulated Brillouin scattering and self focusing. These are wellunderstood effects in single mode fiber but are not easily analyzed inmultimode fiber.

The manufacturing of the flat-sided fiber made from glass may beperformed using conventional fiber drawing techniques. The fiber preformmay have a flat-sided core that may be machined to the desired shapesuch as rectangular with the desired aspect ratio. The cladding may beformed in the conventional way by doping the core to have a highindex-of-refraction, and/or doping the cladding to have a lowindex-of-refraction. Air cladding may be formed by making the preformwith the desired air region geometry and preserving the cross-sectionalgeometry of the air region during the fiber draw process.

Other implementations are also within the scope of the following claims.

1. An optical system comprising: a blue light source; a spatial lightmodulator (SLM); wherein the blue light source emits light only in arange of wavelengths that preserves an optical characteristic of theSLM.
 2. The system of claim 1 wherein the blue light source comprises alaser.
 3. The system of claim 1 wherein the SLM comprises a liquidcrystal material.
 4. A stereoscopic display system comprising: apolarization-switching light source characterized by a polarizationstate; and a polarization-preserving projector which is illuminated bythe polarization-switching light source.
 5. The system of claim 4wherein the polarization-preserving projector forms a left-eye digitalimage and a right-eye digital image, and the polarization state ischanged in synchronization with an alternating projection of theleft-eye digital image and the right-eye digital image.
 6. Astereoscopic projection system comprising: a first infrared laser; afirst gain module that amplifies a light beam from the first infraredlaser; a first second-harmonic generator (SHG) that frequency doubles alight beam from the first gain module; a first optical parametricamplifier (OPO) that parametrically amplifies a light beam from thefirst SHG; a second SHG that frequency doubles a first light beam fromthe first OPO; a third SHG that frequency doubles a second light beamfrom the first OPO; a second infrared laser; a second gain module thatamplifies a light beam from the second infrared laser; and a fourth SHGthat frequency doubles a light beam from the second gain module; whereinpart of the light beam from the first SHG passes through the first OPOto form a remaining light beam, the remaining light beam has a firstwavelength of green light, a light beam from the second SHG has a firstwavelength of red light, a light beam from the third SHG has a firstwavelength of blue light; and a light beam from the fourth SHG has asecond wavelength of green light.
 7. The system of claim 6 wherein theremaining light beam, the light beam from the second SHG, and the lightbeam from the third SHG combine to form an image that is directed to oneeye of a viewer and not directed to the other eye of the viewer.
 8. Thesystem of claim 6 further comprising: a switch that switches the lightbeam from the first SHG; a second OPO that parametrically amplifies thelight beam from the first SHG; a fifth SHG that frequency doubles afirst light beam from the second OPO; and a sixth SHG that frequencydoubles a second light beam from the second OPO; wherein the switchsends the light beam from the first SHG alternately to the first OPO andthe second OPO, and a light beam from the fifth SHG has a secondwavelength of red light, and a light beam from the sixth SHG has secondwavelength of blue light.
 9. The system of claim 6 further comprising: athird infrared laser; a third gain module that amplifies a light beamfrom the third infrared laser; a fifth SHG that frequency doubles alight beam from the third gain module; a second OPO that parametricallyamplifies a light beam from the fifth SHG; a sixth SHG that frequencydoubles a first light beam from the second OPO; and a seventh SHG thatfrequency doubles a second light beam from the second OPO; wherein alight beam from the sixth SHG has a second wavelength of red light, anda light beam from the seventh SHG has a second wavelength of blue light.10. An optical system comprising: a first light source; a second lightsource; and an SLM; wherein the first light source has a first opticaloutput which is processed by a first part of the SLM and the secondlight source has a second optical output which is processed by a secondpart of the SLM.
 11. The system of claim 10 wherein the first lightsource has an etendue lower than 0.1 mm² sr.
 12. The system of claim 10wherein the first part of the SLM is used to form an image for a lefteye of a viewer and the second part of the SLM is used to form an imagefor a right eye of the viewer.
 13. The system of claim 10 wherein thefirst optical output comprises a first wavelength band and the secondoptical output comprises a second wavelength band; the first wavelengthband being distinct from the second wavelength band.
 14. A method ofassembly comprising: placing an alignment plate on a holding plate;inserting a roller and a holding block into the alignment plate;fastening the holding block to the holding plate to hold the roller;fastening the roller to the holding plate; removing the alignment plate;and mating an optical module to the roller on the holding plate.
 15. Themethod of claim 14 further comprising: achieving final optical alignmentwithout further adjustments.
 16. An optical support structurecomprising: a first compartmented support structure adapted to supportoptical modules; and a second compartmented support structure adapted tosupport optical modules; wherein the second compartmented supportstructure is stacked on top of the first compartmented supportstructure.
 17. The structure of claim 16 further comprising: a firstcompartment in the first compartmented support structure; a secondcompartment in the second compartmented support structure; and a holebetween the first compartment and the second compartment that allows abeam of light to pass between the first compartment and the secondcompartment.
 18. The structure of claim 16 further comprising: a thirdcompartment in the second support structure; and a hole between thesecond compartment and the third compartment that allows a beam of lightto pass between the second compartment and the third compartment. 19.The structure of claim 16 further comprising: a kinematic mount on thefirst compartmented support structure; and a kinematic mount on thesecond compartmented support structure; wherein the kinematic mount onthe second compartmented support structure is mated to the kinematicmount on the first compartmented support structure.
 20. An opticalsystem comprising: an OPO; an SHG; a first lens which passes lightbetween the OPO and the SHG; a second lens which passes light betweenthe OPO and the SHG; and a third lens which passes light between the OPOand the SHG.
 21. The system of claim 20 wherein the first lens passes acollimated beam segment to the second lens.
 22. An apparatus comprising:a laser gain slab which carries a main laser beam; a pump laser whichoptically pumps the laser gain slab; and a retroreflective mirrorpositioned adjacent to the laser gain slab; wherein the retroreflectiveminor reflects the main laser beam.
 23. An optical tap comprising: afirst plate; a second plate; and a detector; wherein a first beam oflight enters the first plate, the first beam of light exits the firstplate to form a second beam of light, the second beam of light entersthe second plate, the second beam of light exits the second plate toform a third beam of light, the second plate forms the third beam oflight to be co-linear with the first beam of light, the first beam oflight is reflected from a plate selected from the group consisting ofthe first plate and the second plate to form a fourth beam of light, thefourth beam of light is a small fraction of the first beam of light, andthe fourth beam of light illuminates the detector.
 24. The tap of claim23 further comprising: a third plate; wherein after the first beam oflight reflects from the plate selected from the group consisting of thefirst plate and the second plate, the first beam of light reflects fromthe third plate to form the fourth beam of light.
 25. The tap of claim23 wherein the first plate comprises an uncoated plate of glass.
 26. Anoptical coupler comprising: a first optical fiber; and a despeckler;wherein a first laser light beam illuminates the first optical fiber; anoutput from the first optical fiber illuminates an integrating rod; andan output from the integrating rod illuminates a digital imageprojector.
 27. The coupler of claim 26 further comprising: a secondoptical fiber; wherein a second laser light beam illuminates the secondoptical fiber; and an output from the second optical fiber illuminatesthe despeckler.
 28. The coupler of claim 27 further comprising: a thirdoptical fiber; wherein a third laser light beam illuminates the thirdoptical fiber; an output from the third optical fiber illuminates thedespeckler; the first laser light beam is red; the second laser lightbeam is green; and the third laser light beam is blue.
 29. The couplerof claim 27 wherein the first optical fiber is attached to the secondoptical fiber to form an optical fiber bundle.
 30. An optical systemcomprising: a first laser light source; an optical fiber with a core;and a digital image projector; wherein an output of the first laserlight source illuminates the core, an output of the core illuminates thedigital image projector, and the core has at least one flat side. 31.The system of claim 30 wherein the core has a rectangular cross section.32. The system of claim 31 wherein the output of the first laser lightsource has a polarization direction and the polarization direction isoriented orthogonal to the flat side.